MissingNumber.java

public class MissingNumber {
    public static void main(String[] args){
        int[]arr={1,2,3,4};
        System.out.println(sort(arr));

    }
    static int sort(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++;
            }
        }
    
        for(int index=0;index<arr.length;index++){
            if(arr[index]!=index+1){
                return index+1;
            }
        }
        return arr.length+1;
    }
    static void swap(int []arr,int start, int end){
        int temp=arr[start];
        arr[start]=arr[end];
        arr[end]=temp;
    }
    
    
}