Link: https://leetcode.cn/problems/product-of-array-except-self/

Question

difficulty: mid
adj diff: 3

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.)

左右，乘 2 次就行了。

Code

public int[] productExceptSelf(int[] nums) {
    int len = nums.length;
    int[] ans = new int[len];

    ans[0] = 1;
    int temp = 1;
    for (int i = 1; i < len; i++) {
        // Input: [1, 2, 3, 4]
        // ans  : [1, 1, 2, 6]
        temp *= nums[i - 1];
        ans[i] = temp;
    }
    temp = 1;
    for (int i = len - 2; i >= 0; i--) {
        // Input:= [ 1, 2 , 3, 4]
        // Output: [24, 12, 8, 6]
        temp *= nums[i + 1];
        ans[i] *= temp;
    }

    return ans;
}

238. Product of Array Except Self

Question

Code