STL: lower_bound(·) and upper_bound(·)

Lower Bound

This is a problem I came across in Google interview, and I was caught off guard. The interviewer asked me to implement the lower_bound(·) function in STL. Here is the code:

int lower_bound(int *array, int size, int key)
{
int first = 0, middle;
int half, len;
len = size;

while(len > 0) {
half = len >> 1;
middle = first + half;
if(array[middle] < key) {
first = middle + 1;
len = len-half-1;
}
else
len = half;
}
return first;
}

Upper Bound

Similarly we have:

int upper_bound(int *array, int size, int key)
{
int first = 0, len = size-1;
int half, middle;

while(len > 0){
half = len >> 1;
middle = first + half;
if(array[middle] > key)
len = half;
else{
first = middle + 1;
len = len - half - 1;
}
}
return first;
}