骇人听闻的二分查找（某公司校园招聘笔试试题）（据说只有10%的程序员可以写出没有bug的二分查找）

       二分查找思路很简单，但要把程序写对，却很难，有兴趣的话，可以在网上查一下相关资料，下面给出两种常见的错误：(至于具体错误原因，可以分析程序的执行)

 

错误程序1：

#include <iostream>
using namespace std;

int binarySearch(int a[], int n, int key)
{
int low = 0;
int high = n - 1;
int mid;
while(low < high)        //key可能刚好在low与high相等的地方
{
mid = (low + high)/2;
if(key == a[mid])
return mid;

if(key < a[mid])
high = mid - 1;
else
low = mid + 1;
}

return -1;
}

int main()
{
int a[] = {0, 1, 2};
int n = 3;
int key = 0;
int result = binarySearch(a, n, key);

if(-1 == result)
cout << "no" << endl;
else
cout << "yes! location: " << result + 1 << endl;

return 0;
}

错误程序2：

#include <iostream>
using namespace std;

int binarySearch(int a[], int n, int key)
{
int low = 0;
int high = n - 1;
int mid;
while(low < high)        //可能是死循环
{
mid = (low + high)/2;
if(key == a[mid])
return mid;

if(key < a[mid])
high = mid;
else
low = mid;
}

return -1;
}

int main()
{
int a[] = {0, 1, 2};
int n = 3;
int key = 100;
int result = binarySearch(a, n, key);

if(-1 == result)
cout << "no" << endl;
else
cout << "yes! location: " << result + 1 << endl;

return 0;
}

 正确程序如下：

#include <iostream>
using namespace std;

int binarySearch(int a[], int n, int key)
{
int low = 0;
int high = n - 1;
int mid;
while(low <= high)
{
mid = (low + high)/2;
if(key == a[mid])
return mid;

if(key < a[mid])
high = mid - 1;
else
low = mid + 1;
}

return -1;
}

int main()
{
int a[] = {0, 1, 2};
int n = 3;
int key = 0;
int result = binarySearch(a, n, key);

if(-1 == result)
cout << "no" << endl;
else
cout << "yes! location: " << result + 1 << endl;

return 0;
}