归并排序链表

时间复杂度O(N*logN),空间复杂度O(1)

参考 http://www.cnblogs.com/TenosDoIt/p/3666585.html

#include <iostream>
using namespace std;

struct ListNode {
  int val;
  ListNode *next;
  ListNode(int x) : val(x), next(NULL) {}
};

ListNode *merge(ListNode *head1, ListNode *head2){
  if (head1 == NULL)
    return head2;
  if (head2 == NULL)
    return head1;

  ListNode *head;
  if (head1->val < head2->val){
    head = head1;
    head1 = head1->next;
  }
  else{
    head = head2;
    head2 = head2->next;
  }

  //两个链表不为空
  ListNode *p = head;
  while(head1 != NULL && head2 != NULL){
    if (head1->val < head2->val){
      p->next = head1;
      head1 = head1->next;
    }
    else{
      p->next = head2;
      head2 = head2->next;
    }
    p = p->next;
  }

  //一个链表不为空
  if (head1 != NULL){
    p->next = head1;
  }
  else if (head2 != NULL){
    p->next = head2;
  }

  return head;
}

ListNode *merge_sort_list(ListNode *head){
  if (head == NULL || head->next == NULL){  //0个或1个结点
    return head;
  }

  //找到中间结点
  ListNode *fast = head;
  ListNode *slow = head;
  while(fast->next != NULL && fast->next->next != NULL){
    fast = fast->next->next;
    slow = slow->next;
  }
  ListNode *fast_start = slow->next;
  slow->next = NULL;

  fast = merge_sort_list(fast_start); //后半段
  slow = merge_sort_list(head); //前半段
  return merge(fast, slow);
}


int main(){
  ListNode *head = new ListNode(0);
  ListNode *node1 = new ListNode(3);
  ListNode *node2 = new ListNode(1);
  ListNode *node3 = new ListNode(2);
  ListNode *node4 = new ListNode(6);
  ListNode *node5 = new ListNode(5);
  ListNode *node6 = new ListNode(4);

  head->next = node1;
  node1->next = node2;
  node2->next = node3;
  node3->next = node4;
  node4->next = node5;
  node5->next = node6;
  ListNode *p = head;
  while(p){
    cout << p->val << '\t';
    p = p->next;
  }
  cout << endl;

  ListNode *result = merge_sort_list(head);

  p = result;
  while(p){
    cout << p->val << '\t';
    p = p->next;
  }
  cout << endl;

  int ttt = 0;
  return 0;
}