HàPhan 河

Product of Array Except Self

Given an array nums of `n` integers where `n > 1`, return an array `output` such that `output[i]` is equal to the product of all the elements of `nums` except `nums[i]`.

Example:

``````Input:  [1,2,3,4]
Output: [24,12,8,6]
``````

Constraint: It's guaranteed that the product of the elements of any prefix or suffix of the array (including the whole array) fits in a 32 bit integer.

Note: Please solve it without division and in `O(n)`.

Could you solve it with constant space complexity? (The output array does not count as extra space for the purpose of space complexity analysis.)

Solution

if we can use division, it's simple
Apply same technique as `prefix sum`, we calculate both `prefix product` and `suffix product`, product at `i` is `prefix[i - 1] * suffix[i + 1]`

``````class Solution {
func productExceptSelf(_ nums: [Int]) -> [Int] {

var prefix = Array(repeating: 1, count: nums.count)
var suffix = Array(repeating: 1, count: nums.count)

let lastIndex = nums.count - 1

prefix[0] = nums[0]
suffix[lastIndex] = nums[lastIndex]

for i in 1...lastIndex {
prefix[i] = prefix[i - 1] * nums[i]
suffix[lastIndex - i] = suffix[lastIndex - i + 1] * nums[lastIndex - i]
}

var res = [Int]()
res.append(suffix[1])

for i in 1..<nums.count-1 {
res.append(prefix[i-1] * suffix[i+1])
}

res.append(prefix[nums.count - 2])

return res
}
}
``````

Constant space solution: updating...