[Leetcode 21] 21 Merge Two Sorted Lists

Problem:

Merge two sorted linked lists and return it as a new list. The new list should be made by splicing together the nodes of the first two lists.

 

Analysis:

like merge sort.

Time complexity is O(m+n), Space complexity is O(m+n)

 

Code:

 1 /**
 2  * Definition for singly-linked list.
 3  * public class ListNode {
 4  *     int val;
 5  *     ListNode next;
 6  *     ListNode(int x) {
 7  *         val = x;
 8  *         next = null;
 9  *     }
10  * }
11  */
12 public class Solution {
13     public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
14         // Start typing your Java solution below
15         // DO NOT write main() function
16         if (l1 == null) return l2;
17         if (l2 == null) return l1;
18         
19         ListNode head = null, tail=null;
20         while (l1 != null && l2 != null) {
21             if (l1.val < l2.val) {
22                 if (head == null) {
23                     head = l1;
24                 } else {
25                     tail.next = l1;
26                 }
27                 
28                 tail = l1;
29                 l1 = l1.next;
30             } else {
31                 if (head == null) {
32                     head = l2;
33                 } else {
34                     tail.next = l2;
35                 }
36                 
37                 tail = l2;
38                 l2 = l2.next;
39             }
40         }
41         
42         while (l1 != null) {
43             tail.next = l1;
44             tail = l1;
45             l1 = l1.next;
46         }
47         
48         while (l2 != null) {
49             tail.next = l2;
50             tail = l2;
51             l2 = l2.next;
52         }
53         
54         return head;
55     }
56 }
View Code

 

A highly optimized version:

 1 /**
 2  * Definition for singly-linked list.
 3  * public class ListNode {
 4  *     int val;
 5  *     ListNode next;
 6  *     ListNode(int x) {
 7  *         val = x;
 8  *         next = null;
 9  *     }
10  * }
11  */
12 public class Solution {
13     public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
14         // Start typing your Java solution below
15         // DO NOT write main() function
16         
17         ListNode head = new ListNode(0), tail=head;
18         while (l1 != null && l2 != null) {
19             if (l1.val < l2.val) {     
20                 tail.next = l1;
21                 l1 = l1.next;
22             } else {
23                 tail.next = l2;
24                 l2 = l2.next;
25             }
26             
27             tail = tail.next;
28         }
29         
30        if (l1 != null) tail.next = l1;
31        if (l2 != null) tail.next = l2;
32         
33         return head.next;
34     }
35 }
View Code

 

Attention:

posted on 2013-05-19 06:38  freeneng  阅读(135)  评论(0)  编辑  收藏  举报

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