Find the Duplicate Number

class Solution {
public:
int findDuplicate(vector<int>& nums) {
int n = nums.size();
int s = 1, e = n;
while (s < e) {
int m = s + (e - s) / 2;
int count = 0;
for (int i = 0; i < n; i++) {
if (nums[i] <= m) {
count++;
}
}
if (count > m) {
e = m;
} else {
s = m + 1;
}
}
return s;
}
};