SUMMARY: UndefinedBehaviorSanitizer - c++

Question

Trying to solve Odd Even Linked List question.
Given the head of a singly linked list, group all the nodes with odd indices together followed by the nodes with even indices, and return the reordered list.
My try:
/**
* 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* oddEvenList(ListNode* head) {
if (head == nullptr || head->next == nullptr) return head;
int n = 1;
ListNode* l = nullptr;
ListNode *u = l;
ListNode* r = r;
ListNode* ru = nullptr;
while(head){
ListNode* c = new ListNode(head->val);
if(n%2){
if(r == nullptr){
r = c;
}
else{
r->next = c;
r = r->next;
}
}
else{
if(l == nullptr){
l = c;
}
else{
l->next = c;
l = l->next;
}
}
n++;
head=head->next;
}
l->next = ru;
return u;
}
};
But getting the below error:
Line 27: Char 24: runtime error: member access within misaligned address 0x9ddfea08eb382d69 for type 'ListNode', which requires 8 byte alignment (solution.cpp)
0x9ddfea08eb382d69: note: pointer points here
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior prog_joined.cpp:36:24
What does the error mean and to solve it.
Link: https://leetcode.com/problems/odd-even-linked-list/

Answer 1

The problem is (once we fix the typo):
ListNode *l = nullptr;
ListNode *u = l;
...
return u;
u is always nullptr. One quick hack would be:
ListNode *l = nullptr;
ListNode **u = &l;
ListNode *r = nullptr;
ListNode **ru = &r;
...
l->next = *ru;
return *u;
Or you could try:
if(r == nullptr){
r = c;
ru = r; // Add this
}
...
if(l == nullptr){
l = c;
u = l; // Add this
}

Answer 2

Please check the code with comments marked as // CHANGE HERE
Tested here: https://godbolt.org/z/c3e5M739d
#include <iostream>
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* oddEvenList(ListNode* head) {
if (head == nullptr || head->next == nullptr) return head;
int n = 1;
// CHANGE HERE: set all to nullptr
ListNode* l = nullptr;
ListNode* u = nullptr;
ListNode* r = nullptr;
ListNode* ru = nullptr;
while(head){
// std::cout << head->val << " ";
ListNode* c = new ListNode(head->val);
if(n%2){
// CHANGE HERE: replace r with ru
if(ru == nullptr){
r = c;
// CHANGE HERE: set ru
ru = r;
}
else{
r->next = c;
r = r->next;
}
}
else{
// CHANGE HERE: replace l with u
if(u == nullptr){
l = c;
// CHANGE HERE: set u
u = l;
}
else{
l->next = c;
l = l->next;
}
}
n++;
head=head->next;
}
l->next = ru;
return u;
}
};
int main()
{
ListNode* node = new ListNode(1);
node->next = new ListNode(2);
node->next->next = new ListNode(3);
node->next->next->next = new ListNode(4);
node->next->next->next->next = new ListNode(5);
node->next->next->next->next->next = new ListNode(6);
Solution s;
ListNode* l = s.oddEvenList(node);
ListNode* q = l;
while (q)
{
std::cout << q->val << " ";
q = q->next;
}
}