MissingNumber.java

import javax.sound.midi.Soundbank;

class MissingNumber {
    public static void main(String[] args) {
        int [] arr = {1,0,3};
        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 missing number
        for(int index = 0; index< arr.length;index++){
            if(arr[index] != index){
                return index;
            }
        }
        //case 2
        return arr.length;
    }
        static void swap ( int[] arr, int first, int second){
            int temp = arr[first];
            arr[first] = arr[second];
            arr[second] = temp;
        }
    }