Question

Formatted question description: https://leetcode.ca/all/293.html

 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:
 (input string可以有'-'，比如"+++--++")

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

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

Algorithm

Start traversing from the second letter, each time it is judged whether the current letter is +, and the previous letter is +, if both are plus, the reversed string will be stored in the result.

Code

import java.util.ArrayList;
import java.util.List;

public class Flip_Game {
    public class Solution {
        public List<String> generatePossibleNextMoves(String s) {
            List<String> result = new ArrayList<>();
            char[] arr = s.toCharArray();

            // solution-1: restore
            for(int i = 1; i < s.length(); i++) {
                if(arr[i] == '+' && arr[i - 1] == '+') {
                    arr[i] = '-';
                    arr[i - 1] = '-';
                    result.add(String.valueOf(arr));
                    arr[i] = '+'; // @note: need to resulttore
                    arr[i - 1] = '+';
                }
            }

            // solution-2: just concatenate, no restore
            for(int i = 1; i < s.length(); i++) {
                if(arr[i] == '+' && arr[i - 1] == '+') {
                    result.add(s.substring(0, i - 1) + "--" + s.substring(i + 1));
                }
            }

            return result;
        }
    }
}

############

class Solution {
    public List<String> generatePossibleNextMoves(String currentState) {
        char[] cs = currentState.toCharArray();
        List<String> ans = new ArrayList<>();
        for (int i = 0; i < cs.length - 1; ++i) {
            if (cs[i] == '+' && cs[i + 1] == '+') {
                cs[i] = '-';
                cs[i + 1] = '-';
                ans.add(String.valueOf(cs));
                cs[i] = '+';
                cs[i + 1] = '+';
            }
        }
        return ans;
    }
}

// OJ: https://leetcode.com/problems/flip-game/
// Time: O(N)
// Space: O(1)
class Solution {
public:
    vector<string> generatePossibleNextMoves(string s) {
        vector<string> ans;
        for (int i = 1, N = s.size(); i < N; ++i) {
            if (s[i] != '+' || s[i - 1] != '+') continue;
            ans.push_back(s);
            ans.back()[i] = ans.back()[i - 1] = '-';
        }
        return ans;
    }
};

class Solution(object):
  def generatePossibleNextMoves(self, s):
    """
    :type s: str
    :rtype: List[str]
    """
    ans = []
    for i in range(0, len(s) - 1):
      if s[i:i + 2] == "++":
        ans.append(s[:i] + "--" + s[i + 2:])
    return ans

############

class Solution:
    def generatePossibleNextMoves(self, currentState: str) -> List[str]:
        s = list(currentState)
        ans = []
        for i, c in enumerate(s[:-1]):
            if c == "+" and s[i + 1] == "+":
                s[i] = s[i + 1] = "-"
                ans.append("".join(s))
                s[i] = s[i + 1] = "+"
        return ans

func generatePossibleNextMoves(currentState string) []string {
	ans := []string{}
	cs := []byte(currentState)
	for i, c := range cs[1:] {
		if c == '+' && cs[i] == '+' {
			cs[i], cs[i+1] = '-', '-'
			ans = append(ans, string(cs))
			cs[i], cs[i+1] = '+', '+'
		}
	}
	return ans
}

293 - Flip Game

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