FindDuplicateNumber.java

public class FindDuplicateNumber {

    public static int summationRes(int[] array, int len) {
        for (int i = 0; i < len; i++) {
            array[array[i] % len] += len;
            if (array[array[i] % len] / len == 2)
                return array[i] % len;
        }

        return -999;
    }

    public static int cycleDetectAlgo(int[] array, int len) {
        int fast = len - 1;
        int slow = len - 1;

        while (true) {
            slow = array[slow];
            fast = array[array[fast]];

            if (slow == fast)
                break;
        }

        return array[slow];
    }

    public static void main(String[] args) {
        int[] array = {1, 2, 3, 1, 4, 4};

        System.out.println(cycleDetectAlgo(array, array.length)); // 4
        System.out.println(summationRes(array, array.length)); // 1
    }
}