[Sword finger offer] _16 Build product array

Title description

Given an array A [0,1, ..., n-1], please construct an array B [0,1, ..., n-1], where the elements in B are B[i]=A[0]*A[1]*...*A[i-1]*A[i+1]*...*A[n-1]. You cannot use division.

Problem-solving ideas

Set the array size to 5.

For the first for loop

第一步：b[0] = 1;
第二步：b[1] = b[0] * a[0] = a[0]
第三步：b[2] = b[1] * a[1] = a[0] * a[1];
第四步：b[3] = b[2] * a[2] = a[0] * a[1] * a[2];
第五步：b[4] = b[3] * a[3] = a[0] * a[1] * a[2] * a[3];

For the second for loop

first step

temp *= a[4] = a[4]; 
b[3] = b[3] * temp = a[0] * a[1] * a[2] * a[4];

Second step

temp *= a[3] = a[4] * a[3];
b[2] = b[2] * temp = a[0] * a[1] * a[4] * a[3];

third step

temp *= a[2] = a[4] * a[3] * a[2]; 
b[1] = b[1] * temp = a[0] * a[4] * a[3] * a[2];

the fourth step

temp *= a[1] = a[4] * a[3] * a[2] * a[1]; 
b[0] = b[0] * temp = a[4] * a[3] * a[2] * a[1];

Code

class Solution {
public:
    vector<int> multiply(const vector<int>& A) {
        vector<int> res;
        int length1 = A.size();
        if(length1 == 0)
            return res;
        res.push_back(1);
        for(int i=1;i<length1;++i)
            res.push_back(res[i-1]*A[i-1]);
        double temp =1;
        for(int i=length1-1;i>=0;--i)
        {
            res[i]= res[i]*temp;
            temp*=A[i];
        }
        
        return res;
    }
};