238. Product of Array Except Self 由非己元素形成的数组

Given an array nums of n integers where n > 1,  return an array output such 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]

Constraint: It's guaranteed that the product of the elements of any prefix or suffix of the array (including the whole array) fits in a 32-bit integer.

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.)

 

思路：左边走到哪乘到哪，右边只是在补。

 

class Solution {
    public int[] productExceptSelf(int[] nums) {
        //cc
        int[] res = new int[nums.length];
        
        if (nums == null || nums.length == 0)
            return res;
        
        res[0] = 1;
        int right = 1;
        
        //从左往右乘
        for (int i = 1; i < nums.length; i++) {
            res[i] = res[i - 1] * nums[i - 1];
        }
        
        //从右往左补
        for (int j = nums.length - 1; j >= 0; j--) {
            res[j] *= right;
            right *= nums[j];
        }
        
        //返回
        return res;
    }
}