[Codility] Lession 4.1 FrogRiver

Test URL can be found in extension link
Swift solution:

public func solution(X : Int, inout _ A : [Int]) -> Int {
 
    // Make sure this array is not empty
    guard A.count > 0 else { return 0 }
    // Make sure the value of X is valid,otherwise return -1
    guard X >= 0 else { return -1 }
    // If X == 0 means the expected position is 0 ,which the frog currently located.
    if X == 0 { return 0 }
    
    var result = -1
    var validCount = X
    //Define a boolean array to record which position has been covered by the leafs.
    var arr = [Bool](count:X,repeatedValue: false)
    
    for i in 0 ..< A.count {

        if arr[A[i] - 1] == false {
            arr[A[i] - 1] = true
            validCount -= 1
        }
        
        if validCount == 0 {
            result = i
            break
        }
    }
    return result
}