LeetCode Question #22 How to debug recursion problems?

LeetCode 22. Generate Parentheses

Given n pairs of parentheses, write a function to generate all combinations of well-formed parentheses.

For example, given n = 3, a solution set is:

[
  "((()))",
  "(()())",
  "(())()",
  "()(())",
  "()()()"
]

I am not too sure what is going wrong with my code below. I am using dfs to generate all combinations of well formed parenthesis and I do this by keeping track of the number of left brackets remaining and the number of right brackets remaining. However, the number of remaining brackets are different after I return from a recursion call which causes there to be extra brackets.

vector<string> generateParentheses(int n) {
    vector<string> result;
    dfs(result, "", n, n);
    return result;
}

void dfs(vector<string> &result, string s, int num_left, int num_right){
    if(num_left == 0 && num_right == 0){
        result.push_back(s);
    }
    if(num_left > 0){
        dfs(result, s += "(", num_left - 1, num_right);
    }
    if(num_right > 0 && num_right > num_left){
        dfs(result, s += ")", num_left, num_right - 1);
    }
}