[LeetCode]Product of Array Except Self

Product of Array Except Self

Given an array of n integers where n > 1, nums, return an array output such that output[i] is equal to the product of all the elements of nums except nums[i].

Solve it without division and in O(n).

For example, given [1,2,3,4], return [24,12,8,6].

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

不能用除法，那我们分两次进行，一次将第i位之前的乘起来得到a，再把之后的乘起来得到b，a*b即是我们需要的结果。

class Solution {
public:
    vector<int> productExceptSelf(vector<int>& nums) {
        int n = nums.size();
        vector<int> result(n,1);
        int product = 1;
        for(int i=1;i<n;i++)
        {
            product = product*nums[i-1];
            result[i] = product;
        }
        product = 1;
        for(int i=n-2;i>=0;i--)
        {
            product = product*nums[i+1];
            result[i] = result[i]*product;
        }
        return result;
    }
};