20. Valid Parentheses

 1     public boolean isValid(String s) {
 2         if(s == null || s.length() == 0) {
 3             return false;
 4         }
 5         Stack<Character> stack = new Stack<Character>();
 6         for(int i = 0; i < s.length(); i++) {
 7             char c = s.charAt(i);
 8             if(isLeft(c)) {
 9                 stack.push(c);
10             } else{
11                 if(stack.isEmpty()) {
12                     return false;
13                 } else {
14                     char x = stack.pop();
15                     if(!isMatch(x, c)) {
16                         return false;   
17                     }
18                 }
19             }
20         }
21         return stack.isEmpty();
22     }
23     
24     private boolean isLeft(char c) {
25         if(c == ‘{‘ || c == ‘(‘ || c == ‘[‘) {
26             return true;
27         } else {
28             return false;
29         }
30     }
31     
32     private boolean isMatch(char c1, char c2) {
33         if(c1 == ‘(‘ && c2 == ‘)‘) {
34             return true;
35         }
36         if(c1 == ‘[‘ && c2 == ‘]‘) {
37             return true;
38         }
39         if(c1 == ‘{‘ && c2 == ‘}‘) {
40             return true;
41         }
42         return false;
43     }