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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public class Solution {
    public boolean isValid(String s) {
        if(s == null || s.length() == 0) return false;
        Stack<Character> stack = new Stack<Character>();
        for(int i = 0; i< s.length(); i++){
            if(!stack.empty()){
                if(s.charAt(i) == ')' && stack.peek() == '(')
                    stack.pop();
                else if(s.charAt(i) == ']' && stack.peek() == '[')
                    stack.pop();
                else if(s.charAt(i) == '}' && stack.peek() == '{')
                    stack.pop();
                else
                    stack.push(s.charAt(i));
            }else
                stack.push(s.charAt(i));
        }
        return stack.empty();
    }
}