[LeetCode 293] Flip Game

You are playing the following Flip Game with your friend: Given a string that contains only these two characters:+and -, you and your friend take turns to flip two consecutive "++" into "--". The game ends when a person can no longer make a move and therefore the other person will be the winner.

Write a function to compute all possible states of the string after one valid move.

For example, given s = "++++", after one move, it may become one of the following states:

[
  "--++",
  "+--+",
  "++--"
]

If there is no valid move, return an empty list [].

class Solution
{
public:
	list<string> getFlipResult(string s1)
	{
		list<string> res;
		if (s1.length() < 2)
		{
			return res;
		}
		dfs(s1,res,0);
		return res;
	}
	void dfs(string s1, list<string> &res, int len)
	{
		for (int i = len; i < s1.length()-1; i++)
		{
			if (s1[i] == '+'&&s1[i + 1] == '+')
			{
				string t1 = s1.substr(0, i) + "--" + s1.substr(i+2);
				res.push_back(t1);
			}
			else
			{
				dfs(s1,res,i+1);
			}
		}
	}
};