LeetCode 1541. Minimum Insertions to Balance a Parentheses String

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class Solution {
public:
    int minInsertions(string s) {
        // c is used to track the diff between the cnt of '(' and '))'.
        int n = s.size(), c = 0, ans = 0;
        for (int i = 0; i < n; ++i) {
            if (s[i] == '(') {
                ++c; // c should remain nonnegative.
            } else {
                // if there is no one more ')' available, we need to insert a ')'.
                if (i >= n - 1 || s[i + 1] != ')') ++ans;
                // or, consume it.
                else ++i;
                // if there is no enough preceding '(' to match this '))' group, we need to insert a '('.
                if (c <= 0) ++ans, c = 0;
                // or, consume the preceding '('.
                else --c;
            }
        }
        return ans + c * 2;
    }
};