missingNumber.java

package com.saikat;

import java.util.Arrays;

public class missingNumber {
    public static void main(String[] args) {
        int[] arr = {4,2,1,0};
        miss(arr);
        System.out.println(miss(arr));
    }

    static int miss(int[] arr){
        int i = 0;
        while (i < arr.length){
            int correctIndex = arr[i];
            if (arr[i] < arr.length && arr[i] != arr[correctIndex]){
                swap(arr,i,correctIndex);
            }else{
                i++;
            }
        }
        for (int index = 0; index < arr.length; index++) {
            if (arr[index] != index){
                return index;
            }
        }
        return arr.length;
    }


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