921. Minimum Add to Make Parentheses Valid

Medium (Trung bình) C++ 🔗 Xem trên LeetCode

📋 Đề Bài

A parentheses string is valid if and only if:

It is the empty string,
It can be written as AB (A concatenated with B), where A and B are valid strings, or
It can be written as (A), where A is a valid string.

You are given a parentheses string s. In one move, you can insert a parenthesis at any position of the string.

For example, if s = "()))", you can insert an opening parenthesis to be "(()))" or a closing parenthesis to be "())))".

Return the minimum number of moves required to make s valid.

Example 1:

Input: s = "())"
Output: 1

Example 2:

Input: s = "((("
Output: 3

Constraints:

1 <= s.length <= 1000
s[i] is either '(' or ')'.

🧠 Thuật Toán & Kỹ Thuật

Stack (Ngăn xếp)String (Chuỗi)

⏱️ Thời gian O(n)

💾 Không gian O(1)

💻 Lời Giải

C++ 0921-minimum-add-to-make-parentheses-valid.cpp

class Solution {
public:
    int minAddToMakeValid(string s) {
        stack<char> st;
        int ans = 0;
        
        for (char c : s) {
            if (c == '(') {
                st.push(c);
            }
            else {
                if (!st.empty()) {
                    st.pop();
                }
                else {
                    ans++;
                }
            }
        }
        
        return ans + st.size();
    }
};