leetcode 238. Product of Array Except Self

Given an array nums of n integers where n > 1,  return an array outputsuch 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.)

 

很巧妙的解法，先用一个循环累乘该元素左边的所有数，再用一个循环累乘右边的所有数（倒序循环）。两个串联的循环，最后的时间还是O（n）。

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;
    }
}