1using System;
2using System.Linq;
3
4class SpecialArray {
5    public int SpecialArrayFunc(int[] nums) {
6        Array.Sort(nums);
7        int n = nums.Length;
8        for (int x = 0; x <= n; x++) {
9            int count = nums.Count(num => num >= x);
10            if (count == x) {
11                return x;
12            }
13        }
14        return -1;
15    }
16
17    static void Main() {
18        SpecialArray sa = new SpecialArray();
19        int[] nums = {0, 4, 3, 0, 4};
20        Console.WriteLine(sa.SpecialArrayFunc(nums));
21    }
22}

1using System.Linq;

class SpecialArray {
    private int CountGreaterEqual(int[] nums, int target) {
        return nums.Count(num => num >= target);
    }

    public int SpecialArrayFunc(int[] nums) {
        Array.Sort(nums);
        int left = 0, right = nums.Length;
        while (left <= right) {
            int mid = left + (right - left) / 2;
            int count = CountGreaterEqual(nums, mid);
            if (count == mid) {
                return mid;
            } else if (count < mid) {
                right = mid - 1;
            } else {
                left = mid + 1;
            }
        }
        return -1;
    }

    static void Main() {
        SpecialArray sa = new SpecialArray();
        int[] nums = {0, 4, 3, 0, 4};
        Console.WriteLine(sa.SpecialArrayFunc(nums));
    }
}