Lonly Integer in a Array

There are N integers in an array A. All but one integer occurs in pairs. Your task is to find out that number that occurs only once.

Input Format  

The first line of the input contains an integer N indicating number of integers in the array A.

Next line contains N integers each separated by a single space.

Constraints

1 <= N < 100
N % 2 = 1 ( N is an odd number )
0 <= A[i] <= 100, ∀ i ∈ [1, N]

Output Format

Output (S) which is the number that occurs only once.

Sample Input:1

1
1

Sample Output:1

1

Sample Input:2

3
1 1 2

Sample Output:2

2

Sample Input:3

5
0 0 1 2 1

Sample Output:3

2

Explanation

In the first input, we see only 1 element and that element is the answer (1)
In the second input, we see 3 elements, 1 is repeated twice, the only other element 2 is the answer
In the third input, we see 5 elements, 1 and 0 are repeated twice, the other element 2 is the answer

#include <stdio.h>
#include <string.h>
#include <math.h>
#include <stdlib.h>
#include <assert.h>

/* Head ends here */

int lonelyinteger(int a_size, int* a) 
{
    int count[100];
    for(int i=0;i<100;i++)
    count[i]=0;
    
    for(int i=0;i<a_size;i++)
    {
       count[a[i]]+=1;
    }
    
    for(int i=0;i<100;i++)
    {
      if(count[i]==1)
      return i;
    }
        
return -1;
}

/* Tail starts here */

int main() {
    int res;  
    int _a_size, _a_i;
    scanf("%d", &_a_size);
    int _a[_a_size];
    for(_a_i = 0; _a_i < _a_size; _a_i++) { 
        int _a_item;
        scanf("%d", &_a_item);
        
        _a[_a_i] = _a_item;
    }
    
    res = lonelyinteger(_a_size, _a);
    printf("%d", res);
    
    return 0;
}