MissingNumber.java

package com.devi;

// Use cyclic sort for [1,n] or [0,n] range

public class MissingNumber {

    public static void main(String[] args) {
        int[] arr = {3,0,1};
        System.out.println(missingNumber(arr));

    }

    public static int missingNumber(int[] arr) {
        int i = 0;
        while (i < arr.length) {
            int correct = arr[i];
            if (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) {
                    return index;
                }
            }
            return arr.length;
    }
    public static void swap(int[] arr, int first, int second) {
        int temp = arr[first];
        arr[first] = arr[second];
        arr[second] = temp;
    }

}