《Cracking the Coding Interview》——第2章：链表——题目4

2014-03-18 02:27

题目：将一个单链表按照一个值X分为两部分，小于X的部分放在大于等于X的部分之前。

解法：按照值和X的大小，分链表为两条链表，然后连起来成一条。

代码：

 // 2.4 Write code to partition a linked list around a value x, such that all nodes less than x comes before all nodes greater than or equal to x.
 #include <cstdio>
 using namespace std;

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

 class Solution {
 public:
     ListNode* partitionList(ListNode *head, int x) {
         if (head == nullptr) {
             return head;
         }

         ListNode *h1, *t1, *h2, *t2;
         ListNode *ptr;

         h1 = t1 = nullptr;
         h2 = t2 = nullptr;
         ptr = head;
         while (ptr != nullptr) {
             if (ptr->val < x) {
                 if (h1 == nullptr) {
                     h1 = t1 = ptr;
                 } else {
                     t1->next = ptr;
                     t1 = t1->next;
                 }
                 ptr = ptr->next;
                 t1->next = nullptr;
             } else {
                 if (h2 == nullptr) {
                     h2 = t2 = ptr;
                 } else {
                     t2->next = ptr;
                     t2 = t2->next;
                 }
                 ptr = ptr->next;
                 t2->next = nullptr;
             }
         }
         if (h1 == nullptr) {
             return h2;
         } else if (h2 == nullptr) {
             return h1;
         } else {
             t1->next = h2;
             return h1;
         }
     }
 };

 int main()
 {
     int i;
     int n, x;
     int val;
     struct ListNode *head, *ptr;
     Solution sol;

     while (scanf("%d", &n) ==  && n > ) {
         // create a linked list
         ptr = head = nullptr;
         for (i = ; i < n; ++i) {
             scanf("%d", &val);
             if (head == nullptr) {
                 head = ptr = new ListNode(val);
             } else {
                 ptr->next = new ListNode(val);
                 ptr = ptr->next;
             }
         }

         // partition the list around value x.
         scanf("%d", &x);
         head = sol.partitionList(head, x);

         // print the list
         printf("%d", head->val);
         ptr = head->next;
         while (ptr != nullptr) {
             printf("->%d", ptr->val);
             ptr = ptr->next;
         }
         printf("\n");

         // delete the list
         while (head != nullptr) {
             ptr = head->next;
             delete head;
             head = ptr;
         }
     }

     return ;
 }