MergeSort Recursion

// main function 
void mergeSort(int input[],int size){ // 
    if(size == 0 || size == 1) return;  //base case
    int m = size/2; //finding the mid 
    mergeSort(helper(input,0,m),m);  //left array
    mergeSort(helper(input,m,size),size-m);  // right array
    merge(input,0,m,size); // merging the both arrays
}

void merge(int *input,int start,int size1,int size) //merging array in increasing order
{
    int a[size];
    int i=start,j=size1,k=0;
    while(i < size1 && j < size){
        if(input[i] < input[j]){
            a[k++] = input[i++];
        }
        else{
            a[k++] = input[j++];
        }
    }
    while(i < size1){
        a[k++] = input[i++];
    }
    while(j < size){
        a[k++] = input[j++];
    }
    for(int i=0;i<size;i++){
        input[i] = a[i];
    }
}
int* helper(int input[],int s,int e){// this function helps get the half array
    int a[e-s];
    for(int i=s,j=0;i<e;i++,j++){
        a[j] = input[i];
    }
    for(int i=0;i<e-s;i++){
        input[i] = a[i];
    }
    return input;
}

I am not able to debugg this code can anybody tell me why this is not right I divide the two half with the help of helper function and then pass it to the merge to get the output