POJ 1007 DNA Sorting

#include <iostream>
#include <cstring>
#include <cstdio>
#include <algorithm>

using namespace std;

const int MAXN = 110;

struct DNA_Node
{
int num;
char str[MAXN];

DNA_Node()
{
num = 0;
memset(str, 0, sizeof(str));
}
}DNA_Arr[MAXN];

int sum;
char b[MAXN], a[MAXN];

int cmp(const void* a, const void* b)
{
DNA_Node* aa = (DNA_Node *)a;
DNA_Node* bb = (DNA_Node *)b;
return aa->num - bb->num;
}

void Merge_Find_InverseNum(int low, int mid, int high)
{
int i = low, j = mid+1, k = low;
while(i <= mid && j <= high)
{
if(a[i] <= a[j])
b[k++] = a[i++];
else
b[k++] = a[j++], sum += (mid - i + 1);
}

while(i <= mid)
b[k++] = a[i++];
while(j <= high)
b[k++] = a[j++];

for(i = low; i <= high; ++i)
a[i] = b[i];
return ;
}

void Merge_Sort(int low, int high)
{
if(low == high)
return ;
if(low < high)
{
int mid = (low + high) >> 1;
Merge_Sort(low, mid);
Merge_Sort(mid+1, high);
Merge_Find_InverseNum(low, mid, high);
}
}

int main()
{
int len, num;
while(~scanf("%d %d", &len, &num))
{
for(int i = 0; i < num; ++i)
{
cin>>DNA_Arr[i].str;
strcpy(a, DNA_Arr[i].str);
sum = 0;
Merge_Sort(0, len-1);
DNA_Arr[i].num = sum;
}

qsort(DNA_Arr, num, sizeof(DNA_Arr[0]), cmp);
for(int i = 0; i < num; ++i)
cout<<DNA_Arr[i].str<<endl;
}
return 0;
}