Mohit Kumar - 13 days ago 6
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.

``````/**
* struct ListNode {
*     int val;
*     ListNode *next;
*     ListNode(int x) : val(x), next(NULL) {}
* };
*/
ListNode* Solution::mergeTwoLists(ListNode* A, ListNode* B) {

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){
flag = 1;
}

//root->next = el;
root = root->next;
H1 = H1->next;
}else{
root = new ListNode(H2->val);
if (flag = 0){
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;
}

while(start)
{
cout<<start->val<<" ";
start=start->next;
}

}
``````

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

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* 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
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
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;
}