Find the duplicated number in an array of length N

FindDuplicate.java

public class FindDuplicate {

    public static void main(String[] args) {
        FindDuplicate fd = new FindDuplicate();
        int[] nums = new int[]{1, 2, 3, 4, 5, 6, 1};
        assert (fd.find(nums) == 1);
        nums = new int[]{1, 2, 3, 4, 5, 6, 1};
        assert (fd.find(nums) == 1);
        nums = new int[]{2, 2, 3, 4, 5, 6, 1};
        assert (fd.find(nums) == 2);
        nums = new int[]{3, 2, 3, 4, 5, 6, 1};
        assert (fd.find(nums) == 3);
        nums = new int[]{4, 2, 3, 4, 5, 6, 1};
        assert (fd.find(nums) == 4);
        nums = new int[]{5, 2, 3, 4, 5, 6, 1};
        assert (fd.find(nums) == 5);
        nums = new int[]{6, 2, 3, 4, 5, 6, 1};
        assert (fd.find(nums) == 6);
    }

    private int find(int[] nums) {
        int start = 1;
        int end = nums.length - 1;
        while (start != end) {
            int mid = start + ((end - start) / 2);
            int count = 0;
            int countMid = 0;
            for (int num : nums) {
                if (num > mid && num <= end) count += 1;
                if (num == mid) countMid += 1;
            }
            if (countMid > 1) return mid;
            if (count > (end - mid)) start = mid + 1;
            else end = mid - 1;
        }
        return start;
    }
}