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.

Example:

Input: 1->2->4, 1->3->4
Output: 1->1->2->3->4->4

88. Merge Sorted Array类似,数据结构不一样,这里是合并链表。

由于是链表,不能像数组一样有从后面往前写的技巧。

解法1:dummy list,新建一个链表,然后两个链表中从头各取一个元素进行比较,小的写入新链表,直到结束,返回dummy.next。

解法2:recursion,代码简洁,但空间复杂度高O(n)

Java:

public class ListNode {
int val;
ListNode next;
ListNode(int x) { val = x; }
}
public class Solution {
public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
ListNode flag = new ListNode(0);
ListNode firstflag = flag;
while (l1 != null && l2 != null) {
if(l1.val < l2.val){
flag.next = l1;
l1 = l1.next;
}else {
flag.next = l2;
l2 = l2.next;
}
flag = flag.next;
}
flag.next = l1 != null ? l1 : l2;
return firstflag.next;
}
}

Java:

public ListNode mergeTwoLists(ListNode l1, ListNode l2){
if(l1 == null) return l2;
if(l2 == null) return l1;
if(l1.val < l2.val){
l1.next = mergeTwoLists(l1.next, l2);
return l1;
} else{
l2.next = mergeTwoLists(l1, l2.next);
return l2;
}
} 

Python: Time: O(n), Space: O(1)

class ListNode(object):
def __init__(self, x):
self.val = x
self.next = None def __repr__(self):
if self:
return "{} -> {}".format(self.val, self.next) class Solution(object):
def mergeTwoLists(self, l1, l2):
curr = dummy = ListNode(0)
while l1 and l2:
if l1.val < l2.val:
curr.next = l1
l1 = l1.next
else:
curr.next = l2
l2 = l2.next
curr = curr.next
curr.next = l1 or l2
return dummy.next

Python: wo

class Solution(object):
def mergeTwoLists(self, l1, l2):
"""
:type l1: ListNode
:type l2: ListNode
:rtype: ListNode
"""
head = ListNode(0)
dummy = head
while l1 or l2:
if l1 and l2:
if l1.val < l2.val:
dummy.next = l1
dummy = dummy.next # required
l1 = l1.next
else:
dummy.next = l2
dummy = dummy.next # required
l2 = l2.next
elif l1:
dummy.next = l1
break
elif l2:
dummy.next = l2
break return head.next

Python: Recursion

class Solution(object):
def mergeLists(head1, head2):
temp = None
if head1 is None:
return head2 if head2 is None:
return head1 if head1.val <= head2.val:
temp = head1
temp.next = mergeLists(head1.next, head2) else:
temp = head2
temp.next = mergeLists(head1, head2.next) return temp  

Python: Recursive, wo, Time: O(n), Space: O(n)

class Solution(object):
def mergeTwoLists(self, l1, l2):
"""
:type l1: ListNode
:type l2: ListNode
:rtype: ListNode
"""
if not l1:
return l2 if not l2:
return l1 if l1.val < l2.val:
l1.next = self.mergeTwoLists(l1.next, l2)
return l1
else:
l2.next = self.mergeTwoLists(l1, l2.next)
return l2  

Python:  in-place, iteratively

def mergeTwoLists(self, l1, l2):
if None in (l1, l2):
return l1 or l2
dummy = cur = ListNode(0)
dummy.next = l1
while l1 and l2:
if l1.val < l2.val:
l1 = l1.next
else:
nxt = cur.next
cur.next = l2
tmp = l2.next
l2.next = nxt
l2 = tmp
cur = cur.next
cur.next = l1 or l2
return dummy.next

C++: Time: O(n), Space: O(1)

/**
* Definition for singly-linked list.
* struct ListNode {
* int val;
* ListNode *next;
* ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
public:
ListNode *mergeTwoLists(ListNode *l1, ListNode *l2) {
ListNode dummy{0};
auto curr = &dummy; while (l1 && l2) {
if (l1->val <= l2->val) {
curr->next = l1;
l1 = l1->next;
} else {
curr->next = l2;
l2 = l2->next;
}
curr = curr->next;
}
curr->next = l1 ? l1 : l2; return dummy.next;
}
};

C++: Recursive

class Solution {
public:
ListNode *mergeTwoLists(ListNode *l1, ListNode *l2) {
if(l1 == NULL) return l2;
if(l2 == NULL) return l1; if(l1->val < l2->val) {
l1->next = mergeTwoLists(l1->next, l2);
return l1;
} else {
l2->next = mergeTwoLists(l2->next, l1);
return l2;
}
}
};

类似题目:

[LeetCode] 23. Merge k Sorted Lists 合并k个有序链表

[LeetCode] 88. Merge Sorted Array 合并有序数组

All LeetCode Questions List 题目汇总

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