Stack

Valid Parentheses (LeetCode 20)

import java.util.*;

public class Main {
    public static boolean isValid(String s) {
        Stack<Character> stack = new Stack<>();
        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();
    }
}