20. Valid Parentheses java solutions

Given a string containing just the characters ‘(‘, ‘)‘, ‘{‘, ‘}‘, ‘[‘ and ‘]‘, determine if the input string is valid.

The brackets must close in the correct order, "()" and "()[]{}" are all valid but "(]" and "([)]" are not.

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 1 public class Solution {
 2     public boolean isValid(String s) {
 3         if(s == null || s.length() == 0) return false;
 4         Stack<Character> stack = new Stack<Character>();
 5         for(int i = 0; i< s.length(); i++){
 6             if(!stack.empty()){
 7                 if(s.charAt(i) == ‘)‘ && stack.peek() == ‘(‘)
 8                     stack.pop();
 9                 else if(s.charAt(i) == ‘]‘ && stack.peek() == ‘[‘)
10                     stack.pop();
11                 else if(s.charAt(i) == ‘}‘ && stack.peek() == ‘{‘)
12                     stack.pop();
13                 else
14                     stack.push(s.charAt(i));
15             }else
16                 stack.push(s.charAt(i));
17         }
18         return stack.empty();
19     }
20 }