nyc qs on binary search! must do!

can someonne highlight my error , it's a wrong answer while the output is 3
#include <iostream>
#include<vector>
#include<algorithm>

using namespace std;

int check(int i, vector<int> vec,int c) {
int count = 1;
int pos = 0;
int last = 0;
int maxDistance = vec.back() - vec.front();
while (pos < (vec.size())) {
if (vec[pos] - vec[last] >= i) {
count++;
maxDistance = min(maxDistance, vec[pos] - vec[last]);
last = pos;
}
pos++;
if (count == c)
return maxDistance;
}
return 0;
}

int main() {
int t;
cin >> t;
for (int i = 0; i < t; i++) {
int pos, c;
cin >> pos >> c;
vector<int> vec;
for (int j = 0; j < pos; j++) {
int p;
cin >> p;
vec.push_back(p);
}
sort(vec.begin(), vec.end());
int start = vec[0];
int end = vec.back();
int score = 0;
while (start < end) {
int mid = start + (end - start) / 2;
score = check(mid, vec, c);
if (score>0) {
start = mid + 1;
}
else {
end = mid -1;
}
}
cout << score << endl;
}
}

is it ncecessary that we have to implement it only under the tag under which the ques is categorised..that is DP??

sort(all(a));
ll low = 0;
ll high = a[n-1]-a[0]+1;
ll mid;
while(low < high)
{
//cout<<low<<" "<<high<<endl;
mid = (low+high+1)/2;
if(check(mid,a,c))
low = mid;
else
high = mid-1;
}
cout<<low;



what is problem with this code plz tell

quora explaination ( https://qr.ae/TWIaVc ) was way too better than topcoder tutorial. One of the very good questions of binary search that I have seen. If anyone gets similar question, please comment here .