415_Add_Strings.java

public class Solution {
 public String addStrings(String num1, String num2) {
 StringBuilder sb = new StringBuilder();
 int carry = 0;
 // condition is great
 // https://leetcode.com/problems/add-strings/discuss/90436/Straightforward-Java-8-main-lines-25ms
 for(int i = num1.length() - 1, j = num2.length() - 1; i >= 0 || j >= 0 || carry == 1; i--, j--){
 int x = i < 0 ? 0 : num1.charAt(i) - '0';
 int y = j < 0 ? 0 : num2.charAt(j) - '0';
 sb.append((x + y + carry) % 10);
 carry = (x + y + carry) / 10;
 }
 return sb.reverse().toString();
 }
}