Problem #238: Product of Array Except Self

Intuition & Strategy Breakdown

Construct output array in two passes: compute prefix products from left to right, then multiply by suffix products from right to left in O(n) time.

Python Solution

class Solution:
    def productExceptSelf(self, nums: list[int]) -> list[int]:
        res = [1] * len(nums)
        prefix = 1
        for i in range(len(nums)):
            res[i] = prefix
            prefix *= nums[i]
        postfix = 1
        for i in range(len(nums) - 1, -1, -1):
            res[i] *= postfix
            postfix *= nums[i]
        return res

Java Solution

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