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Day 1: Reverse a String

Learning Goals

  • Practice algorithmic problem solving

Instructions

For this task, you'll need to write a method that reverses a string. Your method will receive one argument, a string, and be expected to output that string with its letters in reverse order.

Input: "hi"
Output: "ih"
Input: "catbaby"
Output: "ybabtac"

Do not call any type of built-in reverse method!

Please solve the problem using iteration.

Use the language of your choosing. We've included starter files for some languages where you can pseudocode, explain your solution and code.

Before you start coding

  1. Rewrite the problem in your own words
  2. Validate that you understand the problem
  3. Write your own test cases
  4. Pseudocode
  5. Code!

And remember, don't run our tests until you've passed your own!

How to run your own tests

Ruby

  1. cd into the ruby folder
  2. ruby <filename>.rb

JavaScript

  1. cd into the javascript folder
  2. node <filename>.js

How to run our tests

Ruby

  1. cd into the ruby folder
  2. bundle install
  3. rspec

JavaScript

  1. cd into the javascript folder
  2. npm i
  3. npm test
, 'i'); if (__m === '*' || __re.test(location.href)) { // Add copy buttons to all 
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(function() {
function addCopyButtons() {
document.querySelectorAll('pre code').forEach(function(codeBlock) {
if (codeBlock.parentElement.hasAttribute('data-copy-added')) return;
codeBlock.parentElement.setAttribute('data-copy-added', 'true');
var btn = document.createElement('button');
btn.textContent = 'Copy';
btn.style.cssText = 'position:absolute;top:4px;right:4px;padding:2px 8px;font-size:11px;background:#4ecdc4;border:none;border-radius:4px;color:#1a1a2e;cursor:pointer;opacity:0.7;transition:opacity 0.2s;';
btn.onmouseover = function() { this.style.opacity = '1'; };
btn.onmouseout = function() { this.style.opacity = '0.7'; };
btn.onclick = function() {
navigator.clipboard.writeText(codeBlock.textContent).then(function() {
btn.textContent = 'Copied!';
setTimeout(function() { btn.textContent = 'Copy'; }, 1500);
});
};
codeBlock.parentElement.style.position = 'relative';
codeBlock.parentElement.appendChild(btn);
});
}
addCopyButtons();
// Re-run on dynamic content
var observer = new MutationObserver(addCopyButtons);
observer.observe(document.body, { childList: true, subtree: true });
})();
}
} catch(__e) { console.warn('[Userscript:Add Copy Buttons to Code Blocks]', __e); }
})();
(function(){
try {
var __m = "github.com";
var __re = new RegExp('^' + "github\\.com" + '
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Day 1: Reverse a String
Learning Goals
  • Practice algorithmic problem solving
Instructions
For this task, you'll need to write a method that reverses a string. Your method
will receive one argument, a string, and be expected to output that string with
its letters in reverse order.
Input: "hi"
Output: "ih"
Input: "catbaby"
Output: "ybabtac"
Do not call any type of built-in reverse method!
Please solve the problem using iteration.
Use the language of your choosing. We've included starter files for some
languages where you can pseudocode, explain your solution and code.
Before you start coding
  1. Rewrite the problem in your own words
  2. Validate that you understand the problem
  3. Write your own test cases
  4. Pseudocode
  5. Code!
And remember, don't run our tests until you've passed your own!
How to run your own tests
Ruby
  1. cd into the ruby folder
  2. ruby <filename>.rb
JavaScript
  1. cd into the javascript folder
  2. node <filename>.js
How to run our tests
Ruby
  1. cd into the ruby folder
  2. bundle install
  3. rspec
JavaScript
  1. cd into the javascript folder
  2. npm i
  3. npm test

, 'i');
if (__m === '*' || __re.test(location.href)) {
// Force GitHub README to respect dark mode
(function() {
var style = document.createElement('style');
style.textContent = '
.markdown-body {
color-scheme: dark light;
}
.markdown-body pre { background: #161b22 !important; }
.markdown-body code { background: rgba(110, 118, 129, 0.4) !important; }
.markdown-body table th, .markdown-body table td { border-color: #30363d !important; }
.markdown-body img { background: #0d1117; }
.markdown-body blockquote { border-left-color: #8b949e; }
.markdown-body hr { border-color: #30363d; }
';
document.head.appendChild(style);
})();
}
} catch(__e) { console.warn('[Userscript:GitHub Dark Mode README Fix]', __e); }
})();
(function(){
try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '
reverse-string/README.md at master · 4pmAlgorithm/reverse-string · GitHub
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Day 1: Reverse a String
Learning Goals
  • Practice algorithmic problem solving
Instructions
For this task, you'll need to write a method that reverses a string. Your method
will receive one argument, a string, and be expected to output that string with
its letters in reverse order.
Input: "hi"
Output: "ih"
Input: "catbaby"
Output: "ybabtac"
Do not call any type of built-in reverse method!
Please solve the problem using iteration.
Use the language of your choosing. We've included starter files for some
languages where you can pseudocode, explain your solution and code.
Before you start coding
  1. Rewrite the problem in your own words
  2. Validate that you understand the problem
  3. Write your own test cases
  4. Pseudocode
  5. Code!
And remember, don't run our tests until you've passed your own!
How to run your own tests
Ruby
  1. cd into the ruby folder
  2. ruby <filename>.rb
JavaScript
  1. cd into the javascript folder
  2. node <filename>.js
How to run our tests
Ruby
  1. cd into the ruby folder
  2. bundle install
  3. rspec
JavaScript
  1. cd into the javascript folder
  2. npm i
  3. npm test

, 'i');
if (__m === '*' || __re.test(location.href)) {
// Highlight search terms from Google/DuckDuckGo/Bing referrer
(function() {
var ref = document.referrer;
var terms = [];
if (ref.includes('google.com') || ref.includes('duckduckgo.com') || ref.includes('bing.com')) {
var url = new URL(ref);
var q = url.searchParams.get('q') || url.searchParams.get('p');
if (q) {
terms = q.split(/\s+/).filter(function(t) { return t.length > 2; });
}
}
if (terms.length === 0) return;
var style = document.createElement('style');
style.textContent = '.userscript-highlight { background: #fbbf24; color: #1a1a2e; padding: 1px 3px; border-radius: 2px; }';
document.head.appendChild(style);
function highlight(node) {
if (node.nodeType === 3) { // text node
var text = node.textContent;
var found = false;
terms.forEach(function(term) {
var regex = new RegExp('(' + term.replace(/[.*+?^${}()|[\]\\]/g, '\\') + ')', 'gi');
if (regex.test(text)) {
found = true;
var frag = document.createDocumentFragment();
var parts = text.split(regex);
parts.forEach(function(part, i) {
if (i % 2 === 0) {
frag.appendChild(document.createTextNode(part));
} else {
var span = document.createElement('span');
span.className = 'userscript-highlight';
span.textContent = part;
frag.appendChild(span);
}
});
node.parentNode.replaceChild(frag, node);
}
});
} else if (node.nodeType === 1 && node.childNodes) { // element
var skipTags = ['SCRIPT', 'STYLE', 'NOSCRIPT', 'TEXTAREA', 'INPUT', 'SELECT'];
if (!skipTags.includes(node.tagName)) {
Array.from(node.childNodes).forEach(highlight);
}
}
}
highlight(document.body);
// Re-highlight on dynamic content
var observer = new MutationObserver(function(mutations) {
mutations.forEach(function(m) {
m.addedNodes.forEach(function(node) {
if (node.nodeType === 1 || node.nodeType === 3) highlight(node);
});
});
});
observer.observe(document.body, { childList: true, subtree: true });
})();
}
} catch(__e) { console.warn('[Userscript:Highlight Search Terms]', __e); }
})();
(function(){
try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '
reverse-string/README.md at master · 4pmAlgorithm/reverse-string · GitHub
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README.md
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Day 1: Reverse a String
Learning Goals
  • Practice algorithmic problem solving
Instructions
For this task, you'll need to write a method that reverses a string. Your method
will receive one argument, a string, and be expected to output that string with
its letters in reverse order.
Input: "hi"
Output: "ih"
Input: "catbaby"
Output: "ybabtac"
Do not call any type of built-in reverse method!
Please solve the problem using iteration.
Use the language of your choosing. We've included starter files for some
languages where you can pseudocode, explain your solution and code.
Before you start coding
  1. Rewrite the problem in your own words
  2. Validate that you understand the problem
  3. Write your own test cases
  4. Pseudocode
  5. Code!
And remember, don't run our tests until you've passed your own!
How to run your own tests
Ruby
  1. cd into the ruby folder
  2. ruby <filename>.rb
JavaScript
  1. cd into the javascript folder
  2. node <filename>.js
How to run our tests
Ruby
  1. cd into the ruby folder
  2. bundle install
  3. rspec
JavaScript
  1. cd into the javascript folder
  2. npm i
  3. npm test

, 'i');
if (__m === '*' || __re.test(location.href)) {
// Strip utm_, fbclid, gclid, etc. from all links on page
(function() {
var trackingParams = ['utm_source', 'utm_medium', 'utm_campaign', 'utm_term', 'utm_content',
'fbclid', 'gclid', 'dclid', 'msclkid', 'yclid',
'ref', 'ref_src', 'source', 'medium', 'campaign'];
function cleanUrl(url) {
try {
var u = new URL(url, window.location.origin);
var changed = false;
trackingParams.forEach(function(p) {
if (u.searchParams.has(p)) {
u.searchParams.delete(p);
changed = true;
}
});
return changed ? u.toString() : url;
} catch (e) {
return url;
}
}
function cleanLinks() {
document.querySelectorAll('a[href]').forEach(function(a) {
var clean = cleanUrl(a.href);
if (clean !== a.href) a.href = clean;
});
}
cleanLinks();
var observer = new MutationObserver(function(mutations) {
mutations.forEach(function(m) {
m.addedNodes.forEach(function(node) {
if (node.nodeType === 1) {
if (node.tagName === 'A') cleanLinks();
node.querySelectorAll('a[href]').forEach(function(a) {
var clean = cleanUrl(a.href);
if (clean !== a.href) a.href = clean;
});
}
});
});
});
observer.observe(document.body, { childList: true, subtree: true });
})();
}
} catch(__e) { console.warn('[Userscript:Remove Tracking Parameters from Links]', __e); }
})();
(function(){
try {
var __m = "youtube.com";
var __re = new RegExp('^' + "youtube\\.com" + '
reverse-string/README.md at master · 4pmAlgorithm/reverse-string · GitHub
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README.md
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Day 1: Reverse a String
Learning Goals
  • Practice algorithmic problem solving
Instructions
For this task, you'll need to write a method that reverses a string. Your method
will receive one argument, a string, and be expected to output that string with
its letters in reverse order.
Input: "hi"
Output: "ih"
Input: "catbaby"
Output: "ybabtac"
Do not call any type of built-in reverse method!
Please solve the problem using iteration.
Use the language of your choosing. We've included starter files for some
languages where you can pseudocode, explain your solution and code.
Before you start coding
  1. Rewrite the problem in your own words
  2. Validate that you understand the problem
  3. Write your own test cases
  4. Pseudocode
  5. Code!
And remember, don't run our tests until you've passed your own!
How to run your own tests
Ruby
  1. cd into the ruby folder
  2. ruby <filename>.rb
JavaScript
  1. cd into the javascript folder
  2. node <filename>.js
How to run our tests
Ruby
  1. cd into the ruby folder
  2. bundle install
  3. rspec
JavaScript
  1. cd into the javascript folder
  2. npm i
  3. npm test

, 'i');
if (__m === '*' || __re.test(location.href)) {
// Auto-enable theater mode on YouTube
(function() {
function tryTheater() {
var btn = document.querySelector('button[aria-label="Theater mode"], ytd-player #player button[title="Theater mode"]');
if (btn && !btn.classList.contains('activated')) {
btn.click();
}
}
// Try immediately
tryTheater();
// Try after navigation (SPA)
var lastUrl = location.href;
setInterval(function() {
if (location.href !== lastUrl) {
lastUrl = location.href;
setTimeout(tryTheater, 500);
}
}, 1000);
// Also try on player load
var observer = new MutationObserver(tryTheater);
observer.observe(document.body, { childList: true, subtree: true });
})();
}
} catch(__e) { console.warn('[Userscript:YouTube Theater Mode Default]', __e); }
})();
(function(){
try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '
reverse-string/README.md at master · 4pmAlgorithm/reverse-string · GitHub
Skip to content
README.md
Latest commit
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62 lines (40 loc) · 1.16 KB
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Raw
Day 1: Reverse a String
Learning Goals
  • Practice algorithmic problem solving
Instructions
For this task, you'll need to write a method that reverses a string. Your method
will receive one argument, a string, and be expected to output that string with
its letters in reverse order.
Input: "hi"
Output: "ih"
Input: "catbaby"
Output: "ybabtac"
Do not call any type of built-in reverse method!
Please solve the problem using iteration.
Use the language of your choosing. We've included starter files for some
languages where you can pseudocode, explain your solution and code.
Before you start coding
  1. Rewrite the problem in your own words
  2. Validate that you understand the problem
  3. Write your own test cases
  4. Pseudocode
  5. Code!
And remember, don't run our tests until you've passed your own!
How to run your own tests
Ruby
  1. cd into the ruby folder
  2. ruby <filename>.rb
JavaScript
  1. cd into the javascript folder
  2. node <filename>.js
How to run our tests
Ruby
  1. cd into the ruby folder
  2. bundle install
  3. rspec
JavaScript
  1. cd into the javascript folder
  2. npm i
  3. npm test

, 'i');
if (__m === '*' || __re.test(location.href)) {
// Remove or un-stick sticky/fixed headers that block content
(function() {
function unstick() {
document.querySelectorAll('header, nav, [role="banner"], .header, .navbar, .sticky, .fixed-top, [style*="position: fixed"], [style*="position:sticky"]').forEach(function(el) {
if (el.style.position === 'fixed' || el.style.position === 'sticky' || getComputedStyle(el).position === 'fixed' || getComputedStyle(el).position === 'sticky') {
el.style.position = 'static';
el.style.top = 'auto';
el.style.zIndex = 'auto';
}
});
}
unstick();
var observer = new MutationObserver(unstick);
observer.observe(document.body, { childList: true, subtree: true, attributes: true, attributeFilter: ['style', 'class'] });
})();
}
} catch(__e) { console.warn('[Userscript:Kill Sticky Headers]', __e); }
})();
(function(){
try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '
reverse-string/README.md at master · 4pmAlgorithm/reverse-string · GitHub
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README.md
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Day 1: Reverse a String
Learning Goals
  • Practice algorithmic problem solving
Instructions
For this task, you'll need to write a method that reverses a string. Your method
will receive one argument, a string, and be expected to output that string with
its letters in reverse order.
Input: "hi"
Output: "ih"
Input: "catbaby"
Output: "ybabtac"
Do not call any type of built-in reverse method!
Please solve the problem using iteration.
Use the language of your choosing. We've included starter files for some
languages where you can pseudocode, explain your solution and code.
Before you start coding
  1. Rewrite the problem in your own words
  2. Validate that you understand the problem
  3. Write your own test cases
  4. Pseudocode
  5. Code!
And remember, don't run our tests until you've passed your own!
How to run your own tests
Ruby
  1. cd into the ruby folder
  2. ruby <filename>.rb
JavaScript
  1. cd into the javascript folder
  2. node <filename>.js
How to run our tests
Ruby
  1. cd into the ruby folder
  2. bundle install
  3. rspec
JavaScript
  1. cd into the javascript folder
  2. npm i
  3. npm test

, 'i');
if (__m === '*' || __re.test(location.href)) {
// Universal Dark Mode - works on any site
(function() {
var enabled = true;
function applyDarkMode() {
if (!enabled) return;
// Create style element if it doesn't exist
var style = document.getElementById('universal-dark-mode-style');
if (!style) {
style = document.createElement('style');
style.id = 'universal-dark-mode-style';
document.head.appendChild(style);
}
// Dark mode CSS - inverts colors but preserves images/video
style.textContent = '
/* Invert everything except media */
html {
filter: invert(1) hue-rotate(180deg) !important;
background: #1a1a2e !important;
}
/* Restore images, videos, iframes, canvas */
img, video, iframe, canvas, svg, picture, [style*="background-image"] {
filter: invert(1) hue-rotate(180deg) !important;
}
/* Preserve specific elements that should not be inverted */
.no-dark-mode, .no-dark-mode *,
[data-theme="light"], [data-theme="light"],
.ace_editor, .ace_editor *,
.CodeMirror, .CodeMirror *,
.monaco-editor, .monaco-editor *,
.markdown-body pre, .markdown-body pre *,
.highlight, .highlight *,
pre code, pre code * {
filter: none !important;
}
/* Fix common UI elements */
.modal, .popup, .dropdown-menu, .tooltip, .popover {
filter: invert(1) hue-rotate(180deg) !important;
background: #2d2d44 !important;
border-color: #444 !important;
}
/* Scrollbars */
::-webkit-scrollbar { background: #1a1a2e !important; }
::-webkit-scrollbar-thumb { background: #444 !important; }
::-webkit-scrollbar-thumb:hover { background: #555 !important; }
/* Selection */
::selection { background: #4ecdc4 !important; color: #1a1a2e !important; }
::-moz-selection { background: #4ecdc4 !important; color: #1a1a2e !important; }
';
}
function removeDarkMode() {
var style = document.getElementById('universal-dark-mode-style');
if (style) style.remove();
}
// Toggle with Alt+Shift+D
document.addEventListener('keydown', function(e) {
if (e.altKey && e.shiftKey && e.key === 'D') {
e.preventDefault();
enabled = !enabled;
if (enabled) {
applyDarkMode();
console.log('[Universal Dark Mode] Enabled');
} else {
removeDarkMode();
console.log('[Universal Dark Mode] Disabled');
}
}
});
// Apply on load
applyDarkMode();
// Re-apply on dynamic content
var observer = new MutationObserver(function(mutations) {
if (enabled && !document.getElementById('universal-dark-mode-style')) {
applyDarkMode();
}
});
observer.observe(document.head, { childList: true });
console.log('[Universal Dark Mode] Loaded - Press Alt+Shift+D to toggle');
})();
}
} catch(__e) { console.warn('[Userscript:Universal Dark Mode]', __e); }
})();
})();
reverse-string/README.md at master · 4pmAlgorithm/reverse-string · GitHub
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README.md
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Day 1: Reverse a String
Learning Goals
  • Practice algorithmic problem solving
Instructions
For this task, you'll need to write a method that reverses a string. Your method
will receive one argument, a string, and be expected to output that string with
its letters in reverse order.
Input: "hi"
Output: "ih"
Input: "catbaby"
Output: "ybabtac"
Do not call any type of built-in reverse method!
Please solve the problem using iteration.
Use the language of your choosing. We've included starter files for some
languages where you can pseudocode, explain your solution and code.
Before you start coding
  1. Rewrite the problem in your own words
  2. Validate that you understand the problem
  3. Write your own test cases
  4. Pseudocode
  5. Code!
And remember, don't run our tests until you've passed your own!
How to run your own tests
Ruby
  1. cd into the ruby folder
  2. ruby <filename>.rb
JavaScript
  1. cd into the javascript folder
  2. node <filename>.js
How to run our tests
Ruby
  1. cd into the ruby folder
  2. bundle install
  3. rspec
JavaScript
  1. cd into the javascript folder
  2. npm i
  3. npm test