二分查找时间复杂度是logn，实现对排序序列的查找
template <typename T>
T BinarySearch(T a[],int n,T key)
{
int left,right;
int mid;
left = 0;
right = n - 1;
while(left<right)
{
mid = (left + right)/2;
if (key == a[mid])
{
return mid;
}
else if (key > a[mid])
{
left = mid + 1;
}
else
{
right = mid - 1;
}
}
return -1;
}
