Valid Parentheses

classic stack problem.

class Solution {
    public boolean isValid(String s) {
        if (s == null || s.length() == 0) return false;
        if (s.length() % 2 == 1) return false;
        
        Stack<Character> stack = new Stack<>();
        for (char c: s.toCharArray()) {
            System.out.println(c);
            if (c == '(') {
                stack.push(')');
            } else if (c == '[') {
                stack.push(']');
            } else if (c == '{') {
                stack.push('}');
            } else if (!stack.isEmpty() && ((c == ')' && stack.peek() == ')') || (c == ']' && stack.peek() == ']') || (c == '}' && stack.peek() == '}') )){
                stack.pop();
            } else {
                return false;
            }
        }
        return stack.isEmpty();
    }
}