C# 二分查找

using System;

using System.Collections.Generic;

using System.Linq;

using System.Text;

using System.Threading.Tasks;

namespace 二分查找

{

    class Program

    {

        static void Main( string [] args)

        {

            int [] arry = new int [] { 1,2,3,5,6,7,8,9};

            //查找有序数列的 某一个数的下标

            //二分查找又称折半查找，优点是比较次数少，查找速度快，平均性能好；

            //其缺点是要求待查表为有序表，且插入删除困难。

            //因此，折半查找方法适用于不经常变动而查找频繁的有序列表。

            int a = HalfFind(arry,0,arry.Length-1,7);

            Console .WriteLine(a);

        }

       

        //递归写法

    static   int BinSearch( int [] Array, int low, int high, int key)

        {

            while (low <= high)

            {

                int mid = (low + high) / 2;

                if (key == Array[mid])

                return mid;

                else if (key < Array[mid])

                //移动low和high 

                {

                    return BinSearch(Array, low, mid - 1, key);

                }

                else

                {

                    return BinSearch(Array, mid + 1, high, key);

                }

            }

                return -1;

           

              

        }

        static int HalfFind( int []arry , int start, int end, int key) {

            int mid = 0;

//等于号 必须有  只有一个元素时 =

            while (start<=end)

            {

                mid = (start + end) / 2;

                if (key < arry[mid])

                {

                    end = mid - 1;

                }

              else   if (key>arry[mid])

                {

                    start = mid + 1;

                }

                else

                {

                    return mid;

                }

            }

            return -1;

        }

    }

}