Mohit Kumar - 9 months ago 43

C++ Question

Please give a simple solution to problem. I have used the algorithm like mergesort but I am not able return head of the auxiliary Linked list, I created. I have seen other example on stack overflow. But I want to know where is the problem with my code.

`/**`

* Definition for singly-linked list.

* struct ListNode {

* int val;

* ListNode *next;

* ListNode(int x) : val(x), next(NULL) {}

* };

*/

ListNode* Solution::mergeTwoLists(ListNode* A, ListNode* B) {

ListNode* head;

ListNode* root;

ListNode* H1 = A;

ListNode* H2 = B;

int flag = 0;

while (H1 != NULL && H2 != NULL){

if(H1->val < H2->val){

root = new ListNode(H1->val);

//cout << root->val << " ";

if (flag = 0){

head = root;

flag = 1;

}

//root->next = el;

root = root->next;

H1 = H1->next;

}else{

root = new ListNode(H2->val);

if (flag = 0){

head =root;

flag = 1;

}

//cout << root->val << " ";

//root->next = el;

root = root->next;

H2 = H2->next;

}

}

while (H2 != NULL){

root = new ListNode(H2->val);

//cout << root->val << " ";

//root->next = el;

root = root->next;

H2 = H2->next;

}

while (H1 != NULL){

root = new ListNode(H1->val);

//cout << root->val << " ";

//root->next = el;

root = root->next;

H1 = H1->next;

}

ListNode *start=head;

while(start)

{

cout<<start->val<<" ";

start=start->next;

}

return head;

}

I have used cout to know the order, It gives the correct order. I am missing something here. None of the list are NULL

Answer Source

Two issues found in your code. At first the equal operator should be changed to Boolean in two places:

```
if (flag = 0){
```

should be

```
if (flag == 0){
```

Then, a tail node should be kept while traversing the two lists.

I converted the code in this (applying minimum changes) which works:

```
ListNode* mergeTwoLists(ListNode* A, ListNode* B) {
ListNode* head;
ListNode* tail; //<-- a tail is introduced
ListNode* root;
ListNode* H1 = A;
ListNode* H2 = B;
int flag = 0;
while (H1 != NULL && H2 != NULL){
if(H1->val < H2->val){
root = new ListNode(H1->val);
//cout << root->val << " ";
if (flag == 0){ //<-- fixed
head = root;
tail=head;
flag = 1;
}
else
{
tail->next=root;
tail = root;
}
//root->next = el;
//root = root->next;
H1 = H1->next;
}else{
root = new ListNode(H2->val);
if (flag == 0){ //<-- fixed
head =root;
tail=head;
flag = 1;
}
else
{
tail->next=root;
tail = root;
}
//cout << root->val << " ";
//root->next = el;
// root = root->next;
H2 = H2->next;
}
}
while (H2 != NULL){
root = new ListNode(H2->val);
//cout << root->val << " ";
//root->next = el;
tail->next=root;
tail=root;
// root = root->next;
H2 = H2->next;
}
while (H1 != NULL){
root = new ListNode(H1->val);
//cout << root->val << " ";
//root->next = el;
tail->next=root;
tail=root;
//root = root->next;
H1 = H1->next;
}
ListNode *start=head;
while(start)
{
cout<<start->val<<" ";
start=start->next;
}
return head;
}
```