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) {
        if (s.size() % 2 != 0) {
            return false;
        }

        stack<char> brackets;
        for (char c : s) {
            if (c == '(' || c == '[' || c == '{') {
                brackets.push(c);
            } else if (
                !brackets.empty() &&
                (c == ')' && brackets.top() == '(' || c == ']' && brackets.top() == '[' ||
                 c == '}' && brackets.top() == '{')) {
                brackets.pop();
            } else {
                return false;
            }
        }

        return brackets.empty();
    }
};