20. Valid Parentheses

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.

public class Solution {
    public boolean isValid(String s) {
        int len = s.length();
        if(len == 0)
           return true;
        if(len%2 != 0)
           return false;
        
        Stack sk = new Stack();
        for(int i=0;i