Offer_ prove safety net _ to build customer cow product array 2020-2-23

2020-2-23

Links: prove safety offer_ bovine construct in the product array guest net _
Title Description:
Given an array A [0,1, ..., n- 1], please construct an array B [0,1, ..., n- 1], wherein the element B is B [i] = a [0 ] * a [1] * ... * a [i-1] * a [i + 1] * ... * a [n-1]. You can not use the division. (Note: The predetermined B [0] = A [1 ] * A [2] * ... * A [n-1], B [n-1] = A [0] * A [1] * ... * A [n -2];)
ideas:
time complexity of O (N) O (N)
can be B [i] = A [0 ] A [1] ... A [i-1] A [i + 1] ... A [ n-1]. As A [0] A [1] ... A [i-1] and A [i + 1] ... A [n-2] A [n-1] is the product of two parts.
I.e. by the product of the B [i] is divided into two parts A [i] item. Effect is equivalent to a diagonal matrix.

The first for loop is used to calculate the number of the FIG. 1 range, the second for loop is used to calculate the number range of 2 to FIG.

Code:

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