Description

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.

Example:

Input: s = "++++"
Output:
[
"--++",
"+--+",
"++--"
]

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

Solution

Iteration, O(n), S(n)

class Solution {
    public List<String> generatePossibleNextMoves(String s) {
        List<String> res = new ArrayList<>();
        if (s == null || s.length() < 2) {
            return res;
        }
        
        char[] arr = s.toCharArray();
        for (int i = 0; i < arr.length - 1; ++i) {
            if (arr[i] == '+' && arr[i + 1] == '+') {
                arr[i] = '-';
                arr[i + 1] = '-';
                res.add(new String(arr));
                arr[i] = '+';
                arr[i + 1] = '+';
            }
        }
        
        return res;
    }
}