Leetcode 238. Product of Array Except Self

[medium]

Product of Array Except Self - LeetCode

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:

• 2 <= nums.length <= 105
• -30 <= nums[i] <= 30
• The product of any prefix or suffix of nums is guaranteed to fit in a 32-bit integer.

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.)

[Java]

1. create another result array for the answer
2. times all the number’s product forward, and times all the number’s product backward.
3. Example:

nums[1, 2, 3, 4] -> first loop res = [1, 1, 2, 6] -> second loop res = [24, 12, 8, 6]