## 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 twoconsecutive`"++"`

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

`[]`

.

Just check whether two adjacent characters are the same and they are equal to ‘+’.

class Solution { public: vector<string> generatePossibleNextMoves(string s) { vector<string> ans; if(s.size() <= 1) return ans; char prev = s[0]; for(int i = 1; i < s.size(); i++) { char curr = s[i]; if(prev == curr && prev == '+'){ ans.push_back(flip(s, i - 1)); } prev = curr; } return ans; } string flip(string s, int i){ s[i] = s[i + 1] = '-'; return s; } };

## Flip Game II

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 twoconsecutive`"++"`

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 determine if the starting player can guarantee a win.

For example, given

`s = "++++"`

, return true. The starting player can guarantee a win by flipping the middle`"++"`

to become`"+--+"`

.

Follow up:

Derive your algorithm’s runtime complexity.

Actually, we can merge playerMove and enemyMove. But I split them to make the program more clear ans self-explained.

O(n!)

class Solution { public: bool canWin(string s) { return playerMove(s); } //return true if player can win bool playerMove(string s){ vector<int> idxs = findFlips(s); for(int i : idxs){ string news = flip(s, i); if(enemyMove(news) == false){ return true; } } return false; } //return true if enemy can win bool enemyMove(string s){ vector<int> idxs = findFlips(s); for(int i : idxs){ string news = flip(s, i); if(playerMove(news) == false){ return true; } } return false; } vector<int> findFlips(string & s){ vector<int> ans; if(s.size() <= 1) return ans; char prev = s[0]; for(int i = 1; i < s.size(); i++){ char curr = s[i]; if(prev == curr && curr == '+'){ ans.push_back(i - 1); } prev = curr; } return ans; } string flip(string s, int i){ s[i] = s[i + 1] = '-'; return s; } };