DFS study code - 301 Remove Invalid Parentheses - Huahua code

Leetcode301RemoveInvalidParentheses.cpp

// Author: Huahua
// Runtime: 0 ms
class Solution {
public:
  vector<string> removeInvalidParentheses(const string& s) {        
    int l = 0;
    int r = 0;
 
    for (const char ch : s) {
      l += (ch == '(');
      if (l == 0)
        r += (ch == ')');
      else
        l -= (ch == ')');
    }
 
    vector<string> ans;
    dfs(s, 0, l, r, ans);
    return ans;
  }
private:
  bool isValid(const string& s) {
    int count = 0;
    for (const char ch : s) {
      if (ch == '(') ++count;
      if (ch == ')') --count;
      if (count < 0) return false;
    }
    return count == 0;
  }
 
  // l/r: number of left/right parentheses to remove.
  void dfs(const string& s, int start, int l, int r, vector<string>& ans) {
    // Nothing to remove.
    if (l == 0 && r == 0) {
      if (isValid(s)) ans.push_back(s);
      return;
    }
 
    for (int i = start; i < s.length(); ++i) {
      // We only remove the first parenthes if there are consecutive ones to avoid duplications.
      if (i != start && s[i] == s[i - 1]) continue;
 
      if (s[i] == '(' || s[i] == ')') {
        string curr = s;
        curr.erase(i, 1);
        if (r > 0 && s[i] == ')') 
          dfs(curr, i, l, r - 1, ans);
        else if (l > 0 && s[i] == '(')
          dfs(curr, i, l - 1, r, ans);
      }
    }
  }
};