20. Valid Parentheses

1. Description

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.
3. Every close bracket has a corresponding open bracket of the same type.

2. Example

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

3. Constraints

• 1 <= s.length <= $10^4$
• s consists of parentheses only ‘()[]{}’.

4. Solutions

Stack

n = s.size()
Time complexity: O(n)
Space complexity: O(n)

class Solution {
public:
    bool isValid(const string &s) {
        stack<char, vector<char>> brackets;
        for (char letter : s) {
            if (letter == '(') {
                brackets.push(')');
            } else if (letter == '[') {
                brackets.push(']');
            } else if (letter == '{') {
                brackets.push('}');
            } else if (brackets.empty() || brackets.top() != letter) {
                return false;
            } else {
                brackets.pop();
            }
        }

        return brackets.empty();
    }
};