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).

这道题讲道理第一遍写的时候没考虑到有数组中有出现0的情况,后面又仔细一看不能用除法，这里犯的另一个错误是vector的初始化问题。leetcode中文版中有详细的解释。
解法一：

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

解法二：

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

一种可能的出错:

Runtime Error
Line 924: Char 9: runtime error: reference binding to null pointer of type 'int' (stl_vector.h)
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior /usr/bin/../lib/gcc/x86_64-linux-gnu/8/../../../../include/c++/8/bits/stl_vector.h:933:9

这种有可能是将方法一种的如下代码写成了错误形式：

//正确形式：
int elem_size = nums.size();
vector<int> res(elem_size);
vector<int> left(elem_size);
//错误形式：
int elem_size = nums.size();
vector<int> res;
vector<int> left;