Valid Parentheses

public class Solution {
    public boolean isValid(String s) {
        if(s==null || s.length()<2) return false;
        Stack<Character> st = new Stack<Character>();
        for(int i=0;i<s.length();i++){
            if(s.charAt(i)==‘(‘||s.charAt(i)==‘[‘||s.charAt(i)==‘{‘)
                st.push(s.charAt(i));
            else if(st.isEmpty()||!helper(st.pop(), s.charAt(i))){
                return false;
            }
        }
        return st.isEmpty()?true:false;
    }
    public boolean helper(char a, char b){
        switch(b){
            case ‘)‘: 
                if(a!=‘(‘) return false;
                else break;
            case ‘]‘: 
                if(a!=‘[‘) return false;
                else break;
            case ‘}‘: 
                if(a!=‘{‘) return false;
                else break;
        }
        return true;
    }
}