#include <iostream.h>
int BinarySearch(int s[100],int x,int low,int high)
{
   
    if(low>high)
        return -1;
    int mid=(low+high)/2;
    if(x==s[mid])
        return mid;
    else if(x>mid)
        return BinarySearch(s,x,mid+1,high);
    else
        return BinarySearch(s,x,low,mid-1);

}
void main()
{
    int s[100],n,x,i,mid;
    cout<<"输入元素个数n=";
    cin>>n;
    cout<<"输入需查找x=";
    cin>>x;
    cout<<"输入数组为:";
    for(i=1;i<=n;i++)
        cin>>s[i];
    int low=s[1],high=s[n];
   
    mid=BinarySearch(s[100],x,low,high);
    cout<<"该元素位于第"<<mid<<"位"<<endl;
   
}

既然报错，你为什么不贴错误信息？想让别人猜着玩？
你考试时，老师是不是只告诉你不及格，而不讲哪题错了？

额 忘记了  我在调试  不好意思

报错如下
--------------------Configuration: 分治法01 - Win32 Debug--------------------
Compiling...
di gui.cpp
D:\c++\分治法01\di gui.cpp(28) : error C2664: 'BinarySearch' : cannot convert parameter 1 from 'int' to 'int []'
        Conversion from integral type to pointer type requires reinterpret_cast, C-style cast or function-style cast
执行 cl.exe 时出错.

分治法01.exe - 1 error(s), 0 warning(s)

提示说得很清楚了，不能由int转化为int数组
mid=BinarySearch(s[100],x,low,high);改为
mid=BinarySearch(s,x,low,high);