要求：程序接受一个白名单文件（一列整数）作为参数，过滤名单中存在的条目，将其余条目打印

1.寻常实现（利用书中管道和重定向接收输入）

  1. public class BinarySearch{
  2.     public static int rank(int key, int[] a){
  3.         int low = 0;
  4.         int high = a.length - 1;
  5.         while(low <= high){
  6.             int mid = (low + high)/2;
  7.             if(a[mid] < key) low = mid + 1;
  8.             else if(a[mid] > key) high = mid - 1;
  9.             else return a[mid];
  10.         }
  11.         return -1;//无键值，返回-1
  12.     }
  14. pubilc static void main(String[] args){
  15.     int[] whitelist = In.readInts(args[0]);
  16.     Arrays.sort(whitelist);
  17.     while(!StdIn.isEmpty()){
  18.             int key = StdIn.readInt();
  19.             if(rank(key, whitelist) < 0)
  20.                 StdOut.println(key);
  21.     }
  22. }

要点
1.high = a.length - 1；
2.low <= high: 当a[] = {1,3,5},查找5时,当条件只是low < high,无法找到5；
3.mid ± 1： 若没有±1操作，当无键值时，无法有low <= high跳出循环

2.递归实现

  1. public int rank(int low, int high, int key, int[] a){
  2.     int mid = (low + high)/2;
  3.     while(low <= high){
  4.         if(a[mid] == key) return a[mid];
  5.         else if(a[mid] < key) return rank(mid+1, high, key, a);
  6.         else return rank(low, mid-1, key, a);
  7.     }
  8.     return -1;
  9. }

3.运行注意

在存放java文件的文件夹中运行PowerShell，输入cmd命令后执行java BinarySearch tinyW.txt < tinyT.txt,此时tinyW.txt即参数args[0],tinyT.txt中的数由StdIn读取