原题链接在这里：https://leetcode.com/problems/flip-game/description/
题目：
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 [].
题解：
若是连着的"++", 就把这段用"--"替代放到res中.
Note: 当i == s.length()-1走到最后一位时. s.substring(i+1), 不会out of index, 会返回"". 但再大就不行了.
Time Complexity: O(n). Space: O(1) regardless res.
AC Java:
1 public class Solution { 2 public List<String> generatePossibleNextMoves(String s) { 3 List<String> res = new ArrayList<String>(); 4 for(int i = 1; i<s.length(); i++){ 5 if(s.charAt(i) == '+' && s.charAt(i-1) == '+'){ 6 res.add(s.substring(0,i-1) + "--" + s.substring(i+1)); 7 } 8 } 9 return res; 10 } 11 }