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:

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

题解：

class Solution {
public:
  void dfs(int n, int l, int r, vector<string> &ans, string &tmp) {
    if (l >= r && l < n) {
      tmp += '(';
      dfs(n, l + 1, r, ans, tmp);
      tmp.pop_back();
      tmp += ')';
      dfs(n, l, r + 1, ans, tmp);
      tmp.pop_back();
    }
    if (l == n) {
      if (r < n) {
        tmp += ')';
        dfs(n, l, r + 1, ans, tmp);
        tmp.pop_back();
      }
      else {
        ans.push_back(tmp);
      }
    }
  }
  vector<string> generateParenthesis(int n) {
    vector<string> ans;
    string tmp = "(";
    if (n == 0) {
      return ans;
    }
    dfs(n, 1, 0, ans, tmp);
    return ans;
  }
};