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<Character> sk = new Stack<Character>();
        for(int i=0;i<len;i++)
        {
            if(s.charAt(i) == '{')
              sk.push('}');
            else if(s.charAt(i) == '[')
              sk.push(']');
            else if(s.charAt(i) == '(')
              sk.push(')');
            else if(sk.empty()||sk.peek()!=s.charAt(i))
              return false;
            else
              sk.pop();
        }
        return sk.empty();
    }
}