C++：
/**
* Definition for singly-linked list.
* struct ListNode {
* int val;
* ListNode *next;
* ListNode() : val(0), next(nullptr) {}
* ListNode(int x) : val(x), next(nullptr) {}
* ListNode(int x, ListNode *next) : val(x), next(next) {}
* };
*/
class Solution {
public:
ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
ListNode *head = NULL;
ListNode *node = NULL;
while ( l1 || l2 ) {
if ( l1 && l2 ) {
if ( l1 -> val > l2 -> val ) {
if ( node ) {
node -> next = l2;
node = node -> next;
} else {
node = l2;
}
l2 = l2 -> next;
node -> next = NULL;
} else if ( l1 -> val < l2 -> val ) {
if ( node ) {
node -> next = l1;
node = node -> next;
} else {
node = l1;
}
l1 = l1 -> next;
node -> next = NULL;
} else {
// if ( l1 -> val == l2 -> val )
if ( node ) {
node -> next = l1;
node = node -> next;
} else {
node = l1;
}
l1 = l1 -> next;
if ( !head && node ) {
head = node;
}
node -> next = l2;
l2 = l2 -> next;
node = node -> next;
node -> next = NULL;
}
} else if ( l1 ) {
if ( node ) {
node -> next = l1;
node = node -> next;
} else {
node = l1;
}
l1 = l1 -> next;
node -> next = NULL;
} else {
// if ( l2 )
if ( node ) {
node -> next = l2;
node = node -> next;
} else {
node = l2;
}
l2 = l2 -> next;
node -> next = NULL;
}
if ( !head && node ) {
head = node;
}
}
return head;
}
};
