LeetCode-in-All

238. Product of Array Except Self

Medium

Given an integer array nums, return an array answer such that answer[i] is equal to the product of all the elements of nums except nums[i].

The product of any prefix or suffix of nums is guaranteed to fit in a 32-bit integer.

You must write an algorithm that runs in O(n) time and without using the division operation.

Example 1:

Input: nums = [1,2,3,4]

Output: [24,12,8,6]

Example 2:

Input: nums = [-1,1,0,-3,3]

Output: [0,0,9,0,0]

Constraints:

Follow up: Can you solve the problem in O(1) extra space complexity? (The output array does not count as extra space for space complexity analysis.)

Solution

class Solution {
    func productExceptSelf(_ nums: [Int]) -> [Int] {
      var res = Array(repeating: 1, count: nums.count)
      var pre = 1
      for i in 0..<nums.count {
        res[i] *= pre
        pre *= nums[i]
      }
      var post = 1
      for i in (0..<nums.count).reversed() {
        res[i] *= post
        post *= nums[i]
      }
      return res
    }
}