Given an integers array A.Define B[i] = A[0] * ... * A[i-1] * A[i+1] * ... * A[n-1], calculate B without divide Operation.ExampleFor A=[1, 2, 3], B is [6, 3, 2]

非常典型的Forward-Backward Traversal方法：

但是第一次做的時候還是忽略了一些問題：比如A.size()==1時，答案應該是空[]

 1 public class Solution { 2     /** 3      * @param A: Given an integers array A 4      * @return: A Long array B and B[i]= A[0] * ... * A[i-1] * A[i+1] * ... * A[n-1] 5      */ 6     public ArrayList<Long> productExcludeItself(ArrayList<Integer> A) { 7         // write your code 8         ArrayList<Long> res = new ArrayList<Long>(); 9         if (A==null || A.size()==0 || A.size()==1) return res;10         long[] lProduct = new long[A.size()];11         long[] rProduct = new long[A.size()];12         lProduct[0] = 1;13         for (int i=1; i<A.size(); i++) {14             lProduct[i] = lProduct[i-1]*A.get(i-1);15         }16         rProduct[A.size()-1] = 1;17         for (int j=A.size()-2; j>=0; j--) {18             rProduct[j] = rProduct[j+1]*A.get(j+1);19         }20         for (int k=0; k<A.size(); k++) {21             res.add(lProduct[k] * rProduct[k]);22         }23         return res;24     }25 }