238. [LeetCode] Product of Array Except Self

Posted 2021-02-11 WhoDreamsSky

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]

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.)

 

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注释：用两个指针，前面的指针fromBegin比返回的res少乘一个当前的i，后面的指针也是如此。当i到达结尾时，前后都覆盖到了。

#include <cstdio>
#include <vector>
#include<iostream>
using namespace std;


class Solution {
public:
    vector<int> productExceptSelf(vector<int>& nums) {
        int n=nums.size();
        int fromBegin=1;
        int fromLast=1;
        vector<int> res(n,1);
        
        for(int i=0;i<n;i++){
            res[i]*=fromBegin;
            fromBegin*=nums[i];
            res[n-1-i]*=fromLast;
            fromLast*=nums[n-1-i];
        }
        return res;
    }
};
int main(){
    vector<int> temp;
    temp.push_back(2);
    temp.push_back(2);
    temp.push_back(2);
    temp.push_back(2);
    temp.push_back(2);
    temp.push_back(2);
    temp.push_back(2);
    temp.push_back(2);
    Solution test;
    test.productExceptSelf(temp);
    return 0;
}