1608. Special Array With X Elements Greater Than or Equal X

You are given an array nums of non-negative integers. nums is considered special if there exists a number x such that there are exactly x numbers in nums that are greater than or equal to x.

Notice that x does not have to be an element in nums.

Return x if the array is special, otherwise, return -1. It can be proven that if nums is special, the value for x is unique.

class Solution {
public:
    int specialArray(vector<int>& A) {
        sort(begin(A), end(A));
        int l = 0, r = A.size();
        while(l <= r) {
            int m = l + (r - l) / 2;
            int count = end(A) - lower_bound(begin(A), end(A), m);
            if(m == count) return m;
            if(count > m) l = m + 1;
            else r = m - 1;
        }
        return -1;
    }
};