求助，c++ vector使用出现问题

Question

#include
#include

using namespace std;

void merge( vector& a, int first, int mid, int last )
{
vector s1 = { a.begin() + first, a.begin() + mid };
vector s2 = { a.begin() + mid + 1, a.begin() + last };
int i = 0; int j = 0; int k = 0;
int len1 = mid - first + 1;
int len2 = last - mid;
for (; ( i < len1 ) && ( j <= len2 ); ++k)
{
if (s1[ i ] < s2[ j ])
a[ k ] = s1[ i++ ];
else
a[ k ] = s2[ j++ ];

}
while (i++ < len1)
    a[ k++ ] = s1[ i ];
while (j++ < len2)
    a[ k++ ] = s2[ j ];

}

void mergesort( vector& a, int first, int last )
{
if (first < last)
{
int mid = ( first + last ) / 2;
mergesort( a, first, mid );
mergesort( a, mid + 1, last );
merge( a, first, mid, last );
}
}

int main()
{
vector a = { 3, 44, 38, 5, 47, 25, 36, 26, 27, 2, 46, 4, 5, 50,99 };

mergesort( a, 0, a.size()-1 );

for (auto i : a)
    cout << i<<"  ";
cout << endl;

return 0;

}

刚学的归并排序
调试看不懂哪里有问题

Answer 1

程序输出什么地方不对？有没有仔细调试？