Given an array of integers, return a new array such that each element at index i of the new array is the product of all the numbers in the original array except the one at i. For example, if our input was [1, 2, 3, 4, 5], the expected output would be [120, 60, 40, 30, 24]. If our input was [3, 2, 1], the expected output would be [2, 3, 6].

Solution.swift

/*
Given an array of integers, return a new array such that each element at index i of the new array is the product of all the numbers in the original array except the one at i.
For example, if our input was [1, 2, 3, 4, 5], the expected output would be [120, 60, 40, 30, 24]. If our input was [3, 2, 1], the expected output would be [2, 3, 6].
*/
import UIKit
// 01 a1 a2 a3
// a0 01 a2 a3
// a0 a1 01 a3
// a0 a1 a2 01
var array = [1, 2, 3]
let count = array.count

// Solution 1:
var left: [Int] = Array(repeating: 1, count: count)
var right: [Int] = Array(repeating: 1, count: count)
var result: [Int] = Array(repeating: 1, count: count)

for i in (1..<count) {
    left[i] = left[i-1] * array[i-1]
}
left
for i in (0..<count-1).reversed() {
    right[i] = right[i+1] * array[i+1]
}
right
for i in (0..<count) {
    result[i] = left[i] * right[i]
}
result

// Solution 2:
var output: [Int] = Array(repeating: 1, count: count)
var temp = 1
for i in (1..<count) {
    output[i] = temp * array[i-1]
    temp = output[i]
}
temp = 1
for i in (0..<count-1).reversed() {
    output[i] = temp * array[i+1]
    temp = output[i]
}
output