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20. Valid Parentheses

Easy

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

An input string is valid if:

1. Open brackets must be closed by the same type of brackets.
2. Open brackets must be closed in the correct order.

Example 1:

Input: s = “()”

Output: true

Example 2:

Input: s = “()[]{}”

Output: true

Example 3:

Input: s = “(]”

Output: false

Example 4:

Input: s = “([)]”

Output: false

Example 5:

Input: s = “{[]}”

Output: true

Constraints:

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

Solution

using System;
using System.Collections.Generic;

public class Solution {
    public bool IsValid(string s) {
        Stack<char> stack = new Stack<char>();
        for (int i = 0; i < s.Length; i++) {
            char c = s[i];
            if (c == '(' || c == '[' || c == '{') {
                stack.Push(c);
            } else if (c == ')' && stack.Count > 0 && stack.Peek() == '(') {
                stack.Pop();
            } else if (c == '}' && stack.Count > 0 && stack.Peek() == '{') {
                stack.Pop();
            } else if (c == ']' && stack.Count > 0 && stack.Peek() == '[') {
                stack.Pop();
            } else {
                return false;
            }
        }
        return stack.Count == 0;
    }
}

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