FB面经Prepare: Dot Product

Conduct Dot Product of two large Vectors

1. two pointers

2. hashmap

3. 如果没有额外空间，如果一个很大，一个很小，适合scan小的，并且在大的里面做binary search

 package fb;

 public class DotProduct {

     public int dotPro(int[][] v1, int[][] v2) {
         int[][] shortV;
         int[][] longV;
         if (v1.length < v2.length) {
             shortV = v1;
             longV = v2;
         }
         else {
             shortV = v2;
             longV = v1;
         }

         int res = 0;
         for (int i=0; i<shortV.length; i++) {
             int shortIndex = shortV[i][0];
             int shortValue = shortV[i][1];
             int longSeq = binarySearch(longV, shortIndex);
             if (longSeq >= 0) {
                 res += shortValue * longV[longSeq][1];
             }
         }
         return res;
     }

     public int binarySearch(int[][] arr, int target) {
         int l=0, r=arr.length-1;
         while (l <= r) {
             int m = (l+r)/2;
             if (arr[m][0] == target) return m;
             else if (arr[m][0] < target) l = m + 1;
             else r = m - 1;
         }
         return -1;
     }

     /**
      * @param args
      */
     public static void main(String[] args) {
         // TODO Auto-generated method stub
         DotProduct sol = new DotProduct();
         int[][] v2 = new int[][]{{0,2},{1,3},{5,2},{7,1},{10,1}};
         int[][] v1 = new int[][]{{1,6},{7,2}};
         int res = sol.dotPro(v1, v2);
         System.out.println(res);
     }

 }