LeetCode [294] 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 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 determine if the starting player can guarantee a win.

Example:

Input: s = "++++"
Output: true 
Explanation: The starting player can guarantee a win by flipping the middle "++" to become "+--+".

Follow up:
Derive your algorithm's runtime complexity.

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class Solution { public: bool canWin(string s) { int n = s.size(); for(int i=0; i<n-1; ++i){ if(s.substr(i,2)=="++"){ string tmp = s; tmp[i] = '-'; tmp[i+1] = '-'; if (!canWin(tmp)) return true; } } return false; } };

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//Java class Solution { public boolean canWin(String s) { int n = s.length(); for(int i=0; i<n-1; ++i){ StringBuilder sb = new StringBuilder(s); if(sb.substring(i, i+2).equals("++")){ sb.replace(i, i+2, "--"); if(!canWin(sb.toString())) return true; } } return false; } }