Leetcode 238. Product of Array Except Self

题目

链接：https://leetcode.com/problems/product-of-array-except-self/

**Level: ** Medium

Discription:
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 1:

Input:  [1,2,3,4]
Output: [24,12,8,6]

Note:

• Please solve it without division and in O(n).

• Could you solve it with constant space complexity? (The output array does not count as extra space for the purpose of space complexity analysis.)

代码

class Solution {
public:
    vector<int> productExceptSelf(vector<int>& nums) {
        vector<int> ret=nums;
        int temp=1;
        for(int i=0;i<nums.size();i++)
        {
            ret[i]=temp;
            temp*=nums[i]; 
        }
        temp=1;
        for(int i=nums.size()-1;i>=0;i--)
        {
            ret[i]*=temp;
            temp*=nums[i]; 
        }
        return ret;
    }
};

思考

• 算法时间复杂度为O(N),空间复杂度为O(1)，不算输出的ret。
• 比较困难的是这题不允许使用除法，使用两次循环，一次记录该数前面的数的乘积，一次记录该数之后的乘积。