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.

 

public class Solution {
    public boolean isValid(String s) {
        if (s.equals("")) {
            return true;
        }
        
        Stack<Character> stack=new Stack<>();
        stack.push('s');
        for (char c : s.toCharArray()) {
            if (c=='(' || c=='[' || c=='{') {
                stack.push(c);
            }else {
                if (c==')' && stack.pop()!='(') {
                    return false;
                }
                if (c==']' && stack.pop()!='[') {
                    return false;
                }
                if (c=='}' && stack.pop()!='{') {
                    return false;
                }
            }
        }
        if (stack.pop()!='s') {
            return false;
        }
        return true;
    }
}