C++ :

#include <bits/stdc++.h>
using namespace std;

int a[10000010],n,i,j;
int main(){
	cin>>n;
	for(i = 0;i < n;i++){
		cin>>a[i];
	}
	
	sort(a,a+n);
	int c = 0;
	for(i = 0;i < n;i++){
		c++;
		if(i == n - 1 || a[i] != a[i + 1]){
			if(c % 2 == 1){
				cout<<a[i]<<endl;
				break;
			}
			
			c = 0;
		}
	}
}

Pascal :

var a,b:array[1..1000000] of longint;
   i,n:longint;
begin
read(n);
for i:=1 to n do
read(a[i]);
for i:=1 to n do
b[a[i]]:=b[a[i]]+1;
for i:=1 to n do
if b[a[i]] mod 2=1 then  begin write(a[i]);break;end;
end.

Java :

import java.util.*;
public class Main {
	public static void main(String args[]){
		Scanner sc=new Scanner(System.in);
		int n=sc.nextInt();
		int[]a=new int[n];
		for(int i =0;i<n;i++){
			a[i]=sc.nextInt();
		}
		int max=a[0];
		for(int i=1;i<a.length;i++){
			if(max<a[i])
				max=a[i];
		}
		int []b=new int[max+1];
		for(int i=0;i<a.length ;i++){
			b[a[i]]++;
		}
		for(int i=0;i<b.length ;i++){
			if(b[i]%2!=0)
				System.out.print(i);
		}
	}
}

Python :

import math
import sys
n=int(input())
m=list(map(int,input().split()))
d=dict()
for item  in m:
    if item not in d:
        d[item]=1
    else:
        d[item]=d[item]+1

for item in d.keys():
    if d[item]%2!=0:
        print(item)
        sys.exit()