二分查算法

二分查找也称折半查找（Binary Search），它是一种效率较高的查找方法。但是，折半查找要求线性表必须采用顺序存储结构，而且表中元素按关键字有序排列。

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using System;    public class Test {     public static void Main()     {         int[] y = new int[] {1,2,3,4,5,6,7,8,9,10,11,12,13 };         int rr = BinarySearch(y, 0, y.Length - 1, 2);         Console.Write(rr);    //12     }     public static int BinarySearch(int[] arr,int low,int high,int key){         int mid = (low+high)/2;         if(low>high){             return -1;         }         else         {             if(arr[mid]==key){                 return mid;             }             else if(arr[mid]>key)             {                 return BinarySearch(arr,low,mid-1,key);             }             else             {                 return BinarySearch(arr,mid+1,high,key);             }         }     } }

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