Q10: How do you validate balanced brackets and parentheses using a Stack?

Traverse the string character by character. When encountering an opening bracket ((, {, [), push its corresponding matching closing bracket onto a Deque<Character> stack. When encountering a closing bracket, check if the stack is empty or if the popped element matches. Return stack.isEmpty() at the end.

Stack Parentheses Validation

import java.util.ArrayDeque;
import java.util.Deque;

public class ValidParentheses {
    public static boolean isValid(String s) {
        Deque<Character> stack = new ArrayDeque<>();
        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();
    }
}

💡 Pushing expected closing brackets makes matching simple.

Key Takeaways

Push expected closing characters to simplify validation logic.
Return stack.isEmpty() to handle unclosed opening brackets.

Knowledge Check

What is the stack state when processing an invalid input string with unmatched opening brackets like "((("?

Q10: How do you validate balanced brackets and parentheses using a Stack?

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