Leetcode: 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 [].

Understand the problem:
The problem only asks for flipping "++" into "--", NOT -- to ++. So the solution is to move all consecutive "++" into "--".


Code (Java):

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public class Solution {     public List<String> generatePossibleNextMoves(String s) {         List<String> result = new ArrayList<>();         if (s == null || s.length() < 2) {             return result;         }                   for (int i = 0; i < s.length() - 1; i++) {             if (s.charAt(i) == '+' && s.charAt(i + 1) == '+') {                 String s1 = s.substring(0, i);                 String s2 = "--";                 String s3 = s.substring(i + 2);                 String temp = s1 + s2 + s3;                 result.add(temp);             }         }                   return result;     } }