3.二分查找

#include "stdio.h"

int BinarySearch(int a[], int n, int x )//数组，数组长度，key

{    intlow=0,high=n-1;

     while(high >= low){    //待搜区间非空

       int mid = (low+ high )/2;

       if (x == a[mid]) return mid; //查找成功则返回相应的位置

       else  if (x < a[mid]) high =mid-1;

       else low = mid+1;

       }

    return -1;  //查找失败，返回-1

} 

int main()

{

int a[]={1,2,4,6};

int sign = 0;

sign = BinarySearch(a, 4, 3);

printf("%d\n",sign);

}