7. Product of Array Except Self

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 input is generated such that answer[i] 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.)

Approach 1

• First compute prefix and then postfix
• Time: O(n) Space: O(1) since the output array is excluded from space analysis.

class Solution {
    public int[] productExceptSelf(int[] nums) {
        int[] d = new int[nums.length];
        int p = 1;
 
        for (int i = 0; i < nums.length; i++) {
            d[i] = p;
            p *= nums[i];
        }
 
        p = 1;
        for (int i = nums.length - 1; i >= 0; i--) {
            d[i] *= p;
            p *= nums[i];
        }
 
        return d;
    }
}