Problem20.java

import java.util.*;

class Problem20 {
 public static void main(String[] args) {
 System.out.println(new Problem20().isValid("{}[(])"));
 }

 public boolean isValid(String s) {
 if (s.length() <= 0) return true;
 Stack<Character> stack = new Stack<>();
 int i = 0;
 char cur = s.charAt(i);
 while(( cur = s.charAt(i) )!= '\0') {
 switch (cur) {
 case ']':
 if (stack.isEmpty()) return false;
 char c = stack.pop();
 if (c != '[') return false;
 break;
 case '}':
 if (stack.isEmpty()) return false;
 c = stack.pop();
 if (c != '{') return false;
 break;
 case ')':
 if (stack.isEmpty()) return false;
 c = stack.pop();
 if (c != '(') return false;
 break;
 default:
 stack.push(cur);
 } 
 ++i;
 if (i == s.length()) {
 break;
 }
 }

 if (stack.isEmpty()) {
 return true;
 }
 return false;
 }
}