[Algorithm] LeetCode 238.product-of-array-except-self

238. Product of Array Except Self

Medium

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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.)

 

Solution:

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