vector<string> generateParenthesis(int n) {

vector<string> result;

string track;

backtrack(n, n, track, result);

return result;

}

void backtrack(int left, int right, string& track, vector<string>& result) {

// 如果左括号多，那么说明不合法

if (left > right) return;

// 如果左或者右小于 0 也不合法了

if (left < 0 || right < 0) return;

// 如果左右都等于 0 了，那说明所有括号匹配了，添加到 result 中

if (left == 0 && right == 0) {

result.push_back(track);

return;

}

track.push_back('(');

backtrack(left - 1, right, track, result);

track.pop_back();

track.push_back(')');

backtrack(left, right - 1, track, result);

track.pop_back();

}