2860. Happy Students

You are given a 0-indexed integer array nums of length n where n is the total number of students in the class. The class teacher tries to select a group of students so that all the students remain happy.

The ith student will become happy if one of these two conditions is met:

• The student is selected and the total number of selected students is strictly greater than nums[i].
• The student is not selected and the total number of selected students is strictly less than nums[i].

Return the number of ways to select a group of students so that everyone remains happy.

class Solution {
public:
    int countWays(vector<int>& nums) {
        int res = 0;
        sort(begin(nums), end(nums));
        for(int i = 0; i < nums.size(); i++) {
            int now = nums[i], nxt = i + 1 == nums.size() ? INT_MAX : nums[i+1];
            if((i + 1) > now and (i + 1) < nxt) res += 1;
        }
        if(nums[0] != 0) res += 1;
        return res;
    }
};