293. 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:

[
  "--++",
  "+--+",
  "++--"
]

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

Solution：

思路： str中向后找"++" 替换 加到结果
Time Complexity: O(N) Space Complexity: O(1)

Solution Code：

class Solution {
    public List<String> generatePossibleNextMoves(String s) {
        List<String> result = new ArrayList<>();
        
        int i = -1;
        while((i = s.indexOf("++", i + 1)) >= 0) {
            result.add(s.substring(0, i) + "--" + s.substring(i + 2));
        }
        
        return result;
    }
}