Leetcode 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.

Difficulty: Easy

public class Solution {
    public boolean isValid(String s) {
        int len = s.length();
        if(len%2 == 1)
            return false;
        HashMap<Character, Character> hm = new HashMap<Character, Character>();
        hm.put('(', ')');
        hm.put('[', ']');
        hm.put('{', '}');
        Stack<Character> st = new Stack<Character>();
        for(int i = 0; i < len; i++){
            if(st.empty()){
                st.push(s.charAt(i));
            }
            else{
                if(hm.containsKey(st.peek()) && hm.get(st.peek()) == s.charAt(i)){
                    st.pop();
                }
                else{
                    st.push(s.charAt(i));
                }
            }
        }
        
        return st.empty();
    }
}