What's wrong with my merge sort implementation (C++)?

Question

My merger sort implementation is working only partially and even raising seg faults on occasion.

Input: 5 4 3 2 1 Output: 0 1 2 3 4

Input: 7 6 5 4 3 2 1 Output: 0 1 2 3 4 5 6

Input: 4 3 2 1 Output: [some garbage value] 1 2 3

Input: 1 2 3 4 5 67 7 8 Output: [some garbage value] 1 2 3 4 5 7 8

It seems that the odd numbered inputs aren't causing seg faults but the even numbered ones are. Still, in both cases the largest value is not appearing in the output. Any help would be appreciated.

void merge(vector<int>& a, int i, int j)
{
     int b[a.size()];
     int start = i;
     int mid = (i+j)/2;
     int k = mid+1;
     int l = i;
     while(i<=mid && k<=j)
     {
       if(a[i] >= a[k])
         b[l++] = a[k++];
       else
         b[l++] = a[i++];
     }

     if(i>mid)
     {
      while(k<=j)
        b[l++] = a[k++];
     }
     else
     {
      while(i<=mid)
        b[l++] = a[i++];
     }

     for(l=start; l<=j; l++)
       a[l] = b[l];
 }

void merge_sort(vector<int>& a, int l, int u)
{
     int m;
     if(l < u)
     {
          m = (l+u)/2;
          merge_sort(a,l,m);
          merge_sort(a,m+1,u);
          merge(a,l,u);
      }
 }

 //relevant portion in main
   cin >> n;
   cout << "n: " << n << endl;
   a.resize(n);
   for(int j=0; j<n; j++)
     cin >> a[j];

   cout << endl;
   merge_sort(a, 0, a.size());

Answer 1

You shouldn't go up to j ("...<= j...") when j could be a.size(). Try a.size() - 1.