12. Product of Array Except Self

Topic :

arrays

Difficulty :

medium

Problem Link :

problem statement

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

import java.io.*;
import java.util.*; 
class Product_Of_Array_Except_Self
{
   public static void main(String args[]){
    int nums[]={1,2,3,4};
    System.out.println(Arrays.toString(productExceptSelf(nums)));
    }
   static int[] productExceptSelf(int[] nums) {
        int n=nums.length;
        int prefix[]=new int[n]; //stores the prefix product of the elements
        int postfix[]=new int[n];//stores the postfix product of the elements
        int ans[]=new int[n];
        
        int pre=1;
        for(int i=0;i<n;i++)
        {
            prefix[i]=pre*nums[i];
            pre=prefix[i];
        }
        
        int post=1;
        for(int i=n-1;i>=0;i--)
        {
            postfix[i]=post*nums[i];
            post=postfix[i];
        }
        
        ans[0]=postfix[1];
        ans[n-1]=prefix[n-2];
        
        for(int i=1;i<n-1;i++)
        {
            ans[i]=prefix[i-1]*postfix[i+1];
        }
        
        return ans;
    }
}