20. Valid Parentheses

Problem Statement

Easy

Given a string s containing just the characters '(', ')', '{', '}', '[' and ']', determine if the input string is valid.

An input string is valid if:

Open brackets must be closed by the same type of brackets.
Open brackets must be closed in the correct order.
Every close bracket has a corresponding open bracket of the same type.

Example 1:

Input: s = "()"

Output: true

Example 2:

Input: s = "()[]{}"

Output: true

Example 3:

Input: s = "(]"

Output: false

Example 4:

Input: s = "([])"

Output: true

Example 5:

Input: s = "([)]"

Output: false

Constraints:

1 <= s.length <= 10⁴
s consists of parentheses only '()[]{}'.

Java

Source file

class Solution {
    public boolean isValid(String s) {
        Stack<Character> stk = new Stack<>();
        int n = s.length();
        for (int i = 0; i < n; i++) {
            char c = s.charAt(i);
            switch (c) {
                case ')' -> {
                    if (!stk.isEmpty() && stk.peek() == '(')
                        stk.pop();
                    else
                        return false;
                }
                case '}' -> {
                    if (!stk.isEmpty() && stk.peek() == '{')
                        stk.pop();
                    else
                        return false;
                }
                case ']' -> {
                    if (!stk.isEmpty() && stk.peek() == '[')
                        stk.pop();
                    else
                        return false;
                }
                default -> stk.push(c);
            }
        }
        return stk.isEmpty();
    }
}