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 [].

思路：从左到右换两个减号。注意要判断原string是否为加号，如果不是减号就不变。用substring会比较简单。

public class Solution {
    public List<String> generatePossibleNextMoves(String s) {
        List<String> res=new ArrayList<>();
        if(s==null||s.length()<=1)
        {
            return res;
        }
        int i=0;
        int j=1;
        while(j<s.length())
        {
            if(s.charAt(i)=='+'&&s.charAt(j)=='+')
            {
            String m=s.substring(0,i)+"--"+s.substring(j+1);
            res.add(m);
            }
            i++;
            j++;
        }
        return res;
    }
}