// A structure data definition
typedef struct __Student
{char name[16];int score;
}Student;
// Array of students
name :  A      B     C     D
score:  80     70    75    70Stable sort in ascending order:
name :  B      D     C     A
score:  70     70    75    80Unstable sort in ascending order:
name :  D      B     C     A
score:  70     70    75    80

// Split arr[] into two parts [begin,mid), [mid,end)
// and using merge_core to merge this two parts
// Total Time O(nlogn)
void merge_sort(int arr[], int begin, int end)
{if (end-begin < 2) return;int mid = (begin+end)>>1;merge_sort(arr,begin,mid);merge_sort(arr,mid,end);merge_core(arr,begin,mid,end);
}   // Time O(logn)

/* merge core: combine two parts which are sorted in ascending order* arr[]: ..., 1, 4, 8, 2, 3, 7, 9, ...*      begin__|        |__mid      |__end* part1: 1, 4, 8       part2: 2, 3, 7, 9 ** combination:* part1:[1]            [4]       [8]* part2: |   [2]  [3]   |   [7]   |   [9]*        |    |    |    |    |    |    |* tmp  :[1]  [2]  [3]  [4]  [7]  [8]  [9]** at last, copyback tmp to arr[begin,end)* */

void merge_core(int arr[], int begin, int mid, int end)
{int i=begin, j=mid, k=0;int *tmp = (int*)malloc(sizeof(int)*(end-begin));for(; i<mid && j<end; tmp[k++]=(arr[i]<arr[j]?arr[i++]:arr[j++]));for(; i<mid; tmp[k++]=arr[i++]);for(; j<end; tmp[k++]=arr[j++]);for(i=begin, k=0; i<end; arr[i++]=tmp[k++]);free(tmp);
}   // Time O(n), Space O(n)

// Merge sort for single list as ascending order
// single list node define
typedef struct __ListNode
{int val;struct __ListNode *next;
}ListNode;// Merge sort for single list without head node
ListNode *merge_sort(ListNode *head)
{if (head==NULL || head->next==NULL) return head;ListNode *fast, *mid, H;// find mid node between head and endfor (H.next=head, fast=mid=&H; fast && fast->next;){mid = mid->next;fast = fast->next->next;}fast = mid->next;mid->next = NULL;   // cut down mid part from head listmid = fast;head = merge_sort(head);mid = merge_sort(mid);return merge_core(head,mid);
}

// merge single list without head node (ascending order)
ListNode *merge_core(ListNode *i, ListNode *j)
{ListNode H, *p;for (p=&H; i && j; p=p->next){if (i->val < j->val){p->next = i;i = i->next;}else{p->next = j;j = j->next;}}p->next = (i ? i:j);return H.next;
}