leetcode-334-reverse-string.swift

/**

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https://leetcode.com/problems/reverse-string/


Write a function that reverses a string. The input string is given as an array of characters char[].

Do not allocate extra space for another array, you must do this by modifying the input array in-place with O(1) extra memory.

You may assume all the characters consist of printable ascii characters.

 

Example 1:

Input: ["h","e","l","l","o"]
Output: ["o","l","l","e","h"]
Example 2:

Input: ["H","a","n","n","a","h"]
Output: ["h","a","n","n","a","H"]

*/

/**
Big O Annotation
Time complexity O(log n) where n is the amount of elements in reverseString.
The two pointer recursion cuts the work load in half.

Space complexity O(1) we're not using any extra space.
*/
func reverseString(_ s: inout [Character]) {
        
    swapCharacters(
        atLeftPointer: 0,
        andRightPointer: s.count - 1,
        in: &s
    )
}

/**
The goal of this challenge was to use an iterative approach,
so that's what we're doing.
*/
func swapCharacters(atLeftPointer left: Int, 
                    andRightPointer right: Int, 
                    in array: inout [Character]) {
    
    // The base case hits when the pointers cross
    guard left < right else {
        return 
    }
    
    // Until then swap the elements at the pointers
    array.swapAt(left, right)
    
    // And repeat by increasing/decreasing the pointers
    swapCharacters(
        atLeftPointer: left + 1, 
        andRightPointer: right - 1, 
        in: &array
    )
}