import java.util.Arrays;

public class CyclicSort {
        public static int firstMissingPositive(int[] arr) {
            int i = 0;
            while (i < arr.length) {
                int correct = arr[i] - 1;
                if (arr[i] > 0 && arr[i] <= arr.length && arr[i] != arr[correct]) {
                    swap(arr, i , correct);
                } else {
                    i++;
                }
            }

            // search for first missing number
            for (int index = 0; index < arr.length; index++) {
                if (arr[index] != index + 1) {
                    return index + 1;
                }
            }

            // case 2
            return arr.length + 1;
        }

        static void swap(int[] arr, int first, int second) {
            int temp = arr[first];
            arr[first] = arr[second];
            arr[second] = temp;
        }
    }