Algorithmic skills. StrSymmetryPoint.swift

import Foundation
import Glibc

// Solution @ Sergey Leschev, Belarusian State University

// Algorithmic skills. StrSymmetryPoint.

// Write a function:
// class Solution { public int solution(String S); }
// that, given a string S, returns the index (counting from 0) of a character such that the part of the string to the left of that character is a reversal of the part of the string to its right. The function should return −1 if no such index exists.

// Note: reversing an empty string (i.e. a string whose length is zero) gives an empty string.

// For example, given a string:
// "racecar"
// the function should return 3, because the substring to the left of the character "e" at index 3 is "rac", and the one to the right is "car".

// Given a string:
// "x"
// the function should return 0, because both substrings are empty.

// Write an efficient algorithm for the following assumptions:
// the length of string S is within the range [0..2,000,000].

public func solution(_ S: inout String) -> Int {
        let count = S.count
        if count == 1 { return 0 }
        
        if count % 2 == 0 || count == 0 { return -1 }
        let center = count / 2
        
        let left = S.index(S.startIndex, offsetBy: center)
        let right = S.index(S.startIndex, offsetBy: count - center)
        
        let leftSubstring = S[..<left]
        let rightReversedSubstring = String(S[right...].reversed())
        
        return leftSubstring == rightReversedSubstring ? center : -1
}