构建乘积数组

题目描述

给定一个数组A[0,1,...,n-1],请构建一个数组B[0,1,...,n-1],其中B中的元素B[i]=A[0]*A[1]*...*A[i-1]*A[i+1]*...*A[n-1]。不能使用除法。

源代码：

import java.util.ArrayList;
public class Solution {
    public int[] multiply(int[] A) {
        int len=A.length;
        int [] arr=new int[len];
        arr[0] = 1;
        for(int i=1;i<len;i++){
            arr[i] = arr[i-1]*A[i-1];
        }

/*

arr[0] = 1;  

arr[1] = arr[0] *A[0]=     A[0];       

arr[2] = arr[1] *A[1]=      A[0] *A[1];       

arr[3] = arr[2] *A[2]=      A[0] *A[1]*A[2]     

arr[4] = arr[3] *A[3]=      A[0] *A[1]*A[2]*A[3]        

arr[5] = arr[4] *A[4]=         A[0] *A[1]*A[2]*A[3]*A[4]......................        

arr[len-1] = arr[len-2] *A[len-2]=              A[0] *A[1]*A[2]*A[3]*A[4]*.............*A[len-3]*A[len-2]

*/
        int tmp=1;
        for(int i=len-1;i>=0;i--){
            arr[i] = arr[i]*tmp;
            tmp=tmp*A[i];
        }

/*

tmp=1

arr[len-1]=arr[len-1] *tmp=A[0] *A[1]*A[2]*A[3]*A[4]*.............*A[len-3]*A[len-2]*1=A[0] *A[1]*A[2]*A[3]*A[4]*.............*A[len-3]*A[len-2]

tmp=1*A[len-1]

arr[len-2]=arr[len-2] *tmp=             A[0] *A[1]*A[2]*A[3]*A[4]*.............*A[len-3]*A[len-1]

tmp=  A[len-1]*A[len-2]

arr[len-3]=arr[len-3] *tmp=              A[0] *A[1]*A[2]*A[3]*A[4]*.............*A[len-4]*A[len-2]*A[len-1]

tmp=  A[len-1]*A[len-2]*A[len-3]

arr[1] =                   A[0] *A[2]*A[3]*A[4]*.............*A[len-4]*A[len-2]*A[len-1]

tmp=  A[len-1]*A[len-2]*A[len-3]*...................*A[1]

arr[0] = arr[0]*tmp =     arr[0]*A[len-1]*A[len-2]*A[len-3]*...................*A[1] =    A[len-1]*A[len-2]*A[len-3]*...................*A[1]

*/

        return arr;
    }
}