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:

  1. [
  2.   "--++",
  3.   "+--+",
  4.   "++--"
  5. ]

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

Solution:

  1. public class Solution {
  2.   public List<String> generatePossibleNextMoves(String s) {
  3.     List<String> res = new ArrayList<>();
  5.     // sanity check
  6.     if (s == null || s.length() <= 1)
  7.       return res;
  9.     for (int i = 0; i < s.length() - 1; i++)
  10.       if (s.startsWith("++", i))
  11.         res.add(s.substring(0, i) + "--" + s.substring(i + 2));
  13.     return res;
  14.   }
  15. }