func lonelyinteger(a: [Int]) -> Int {
    var dict = [Int: Bool]()
    
    a.forEach { value in
        // will nil the dictionary entry if more than 1 occurrence
        if dict[value] != nil {
            dict[value] = nil
        } else {
            dict[value] = true
        }
    }
    
    // should only be 1 entry left
    return dict.keys.first ?? -1
}

public static int lonelyinteger(List<Integer> arr) {
        HashSet<Integer> set = new HashSet<>();
        HashSet<Integer> uniqueSet = new HashSet<>();
        
        Integer unique = arr.get(0);
        Integer lastUnique = arr.get(0);
        
        for(Integer element: arr){
            if(!set.contains(element)){
                set.add(element);
                uniqueSet.add(element);
            }else{
                uniqueSet.remove(element);
            }
        }
        
        return uniqueSet.stream().findFirst().get();
    }

public static int lonelyinteger(List<int> a)
    {
        int size = a.Count();
        if(size == 1) return a[0];
        int item = a[0];
        int count = 1;
        List<int> buffer = new List<int>{};
        for(int i = 0; i < size; i++){
            
            if(a[i] < -1 || a[i] > 101) throw new Exception("Invalid items");
            
            count = 1;
            if(buffer.Contains(a[i])) {
                count++;
                continue;
            }
            for(int j = i+1; j <= size - 1; j++){ 
                if(a[i] == a[j]){
                    count++;
                    buffer.Add(a[i]);
                    break;
                }
            }
            if(count < 2) item = a[i];
        }
        return item;
    }

def lonelyinteger(a):
    counter = dict()
    for num in a:
        counter[num] = counter.get(num, 0) +1
    return sorted(counter.items(), key= lambda x: x[1])[0][0]

function lonelyinteger(a) {
    let result,i,j;
    for(i=0; i<a.length; i++){
        let count = 0;
        for(j=0; j<a.length; j++){
            if(a[i]==a[j]){
                count++;
            }
        }
        
        if(count==1){
            result = a[i];
        }
    }
    return result
}