Differenza simmetrica

#include <stdio.h>
#include <stdlib.h>

struct List{
    int key;
    struct List* next;
}

typedef struct List* List;

//creazione di un nodo
List newElem(int k){
    List temp= (List)malloc(sizeof(List));
    temp->key= k;
    temp->next= NULL;

    return temp;
}

//inserimento in testa
void addHead(List L, int k){
    List toAdd= newElem(k);
    if(L != NULL){
        toAdd->next= L;
        L= toAdd;
    }
    else L= toAdd;
}

//funzione di ricerca
int search(List L, int k){
    if(L != NULL){
        if(L->key == k) return 1;
        else return search(L->next, k);
    }

    return 0;
}


List diffSimm(List L1, List L2){
    if(L1 && L2){
        List L3= NULL;
        L3->next= diffSimm(L1->next, L2->next);

        if(!search(L1, L2->key))
            addHead(L3, L2->key);

        if(!search(L2, L1->key))
            addHead(L3, L1->key);
    }

    return L3;
}