4-6

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

//     s:
// stack:

//     s: )]
// stack: ])

//     s: )[]
// stack:

//     s:
// stack: )