leetcode-293-flip-game.swift

/**

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https://leetcode.com/problems/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.

Example:

Input: s = "++++"
Output: 
[
  "--++",
  "+--+",
  "++--"
]
Note: If there is no valid move, return an empty list [].

*/

/**
Big O Annotation
Time complexity O(n) where n is the amount of characters in s.
Space complexity O(n) where n is the amount of possible states.
*/
func generatePossibleNextMoves(_ s: String) -> [String] {
        
    // Create an empty states array
    var states: [String] = []

    /**
    Get a character array from s 
    for easier index access
    */
    var s: [Character] = Array(s)
    
    // Iterate through S
    for (index, character) in s.enumerated() {
        
        // Check if the current element is a +
        if character == "+" {
            
            // Get a mutable copy of the array
            var current = s
            
            // Check if the next index is also a +
            if (index + 1) < s.count && s[index + 1] == "+"  {
                
                // Turn both indices to -
                current[index] = "-"
                current[index + 1] = "-"

                // ... and add to the states
                states.append(String(current))
            }
        }
    }
    
    return states
}