238. Product of Array Except Self

Medium

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]

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

Follow up:
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:

1) Construct such a temporary array left[i] which contains the product of all the elements on left side of element i exclude element i.

2) Construct such a temporary array right[i] which contains the product of all the elements on right side of element i exclude element i.

3) get the prod[], which multiply left[] and right[]

    public int[] productExceptSelf(int[] nums) {
        if(nums == null) return null;
        int n = nums.length;
        int[] left = new int[n];
        int[] right = new int[n];
        left[0] = 1;
        for(int i = 1 ; i < n; i++){
            left[i] = nums[i-1] * left[i-1];
        }
        right[n-1] = 1;
        for(int i = n - 2; i>=0; i--){
            right[i] = nums[i+1] * right[i+1];
        }
        int[] res = new int[n];
        for(int i = 0; i < n ; i++){
            res[i] = left[i] * right[i];
        }
        return res;
    }