Help me in solving CHEFDETE problem

My issue

plz give me hint …

My code

#include <iostream>
using namespace std;

int main() {
	// your code goes here
	return 0;

Problem Link: CHEFDETE Problem - CodeChef

since the killer will be reported by nobody so its must not be present in the array so u have to find the numbers that are not present in the array from 1 to N.
ex for N=6 and array is [0,1,1,2,2,3] 4,5,6 are not present is the array so these may be the killer .