二分查找的原始写法如这篇文章所示
如果条件变化一下，比如数组中的元素可能重复，要求返回最小（或最大）的下标。
或者找出数组中第一个大于value（也就是最小的大于value）的元素下标等等，
代码实现会有所不同。
一共分为六种不同情况，下面列出这些变形的二分查找
// 找出第一个与value相等的元素
int firstEqual(int A[], int left, int right, int value)
{
int originleft = left, originright = right;
while (left <= right)
{
int mid = left + ((right - left) >> 1);
if (value > A[mid])
left = mid + 1;
else
right = mid - 1;
}
if (A[left] == value && left >= originleft && right <= originright)
return left;
return -1;
}// 查找第一个大于value的元素
int firstBigger(int A[], int left, int right, int value)
{
while (left <= right)
{
int mid = left + ((right - left) >> 1);
if (value >= A[mid])
left = mid + 1;
else
right = mid - 1;
}
return left;
}// 查找第一个等于或者大于value的元素
int firstEqualOrBigger(int A[], int left, int right, int value)
{
while (left <= right)
{
int mid = left + ((right - left) >> 1);
if (value > A[mid])
left = mid + 1;
else
right = mid - 1;
}
return left;
}// 找出最后一个与value相等的元素
int lastEqual(int A[], int left, int right, int value)
{
int originleft = left, originright = right;
while (left <= right)
{
int mid = left + ((right - left) >> 1);
if (value < A[mid])
right = mid - 1;
else
left = mid + 1;
}
if (A[right] == value && left >= originleft && right <= originright)
return right;
return -1;
}// 查找最后一个小于value的元素
int lastSmaller(int A[], int left, int right, int value)
{
while (left <= right)
{
int mid = left + ((right - left) >> 1);
if (value <= A[mid])
right = mid - 1;
else
left = mid + 1;
}
return right;
}// 查找最后一个等于或者小于value的元素
int lastEqualOrSmaller(int A[], int left, int right, int value)
{
while (left <= right)
{
int mid = left + ((right - left) >> 1);
if (value < A[mid])
right = mid - 1;
else
left = mid + 1;
}
return right;
}