**Problem Statement: CodeChef: Practical coding for everyone**.

I have already solved the problem using a different approach by creating my own function for lower and upper bound but when I am using the in-built function `lower_bound()`

and `upper_bound()`

defined in the C++ STL library I am getting **WA** in the last test-case only.

Below is the function which computes the minimum time as asked in the **problem statement**:

```
static ll_t compute_minimum_time(const std :: vector <std :: pair <ll_t, ll_t>> & contest_schedule, std :: vector <ll_t> & wormhole_open_time, std :: vector <ll_t> & wormhole_close_time) {
std :: sort(wormhole_open_time.begin(), wormhole_open_time.end());
std :: sort(wormhole_close_time.begin(), wormhole_close_time.end());
ll_t min_time = 1e10;
for(const auto & schedule: contest_schedule) {
std :: vector <ll_t> :: iterator t1 = std :: upper_bound(wormhole_open_time.begin(), wormhole_open_time.end(), schedule.first);
if(t1 == wormhole_open_time.begin()) {
continue;
}
// t1 = wormhole_open_time.begin() + (t1 - wormhole_open_time.begin()) - 1;
--t1;
std :: vector <ll_t> :: iterator t2 = std :: lower_bound(wormhole_close_time.begin(), wormhole_close_time.end(), schedule.second);
if(t2 == wormhole_close_time.end()) {
continue;
}
min_time = std :: min(min_time, 1 + ((*t2) - (*t1)));
}
return min_time;
}
```

If you want to refer to the whole source-code then go to: **Full Source-Code**.

I am looking for the test-cases where my code will fail as it is failing to produce **AC** in the very last given case. Moreover, I would appreciate it, if someone can tell me what and where is the bug in my program in detail so that I don’t create the same bug in the future.

**Thanks**