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int length(ListNode* head){
        int ct = 0;
        ListNode* temp = head;
        while(temp != NULL){
            temp = temp->next;
            ct++;
        }
        return ct;
    }
    
ListNode* reverse(ListNode* head){
    ListNode* p = NULL;
    if(head == NULL){
        return NULL;
    }
    if(head->next == NULL){
        return head;
    }
    ListNode* c = head;
    ListNode* n;
    while(c != NULL){
        n = c->next;
        c->next = p;
        p = c;
        c = n;
    }
    return p;
}
 
 
int Solution::lsPalindrome(ListNode* A) {
        ListNode* H = A;
        int n = length(A)/2;
        ListNode* temp = A;
        while(--n){
            temp = temp->next;
        }
        ListNode *t = reverse(temp->next);
        
        while(t != NULL && H!=NULL){
            if(H->val != t->val){
                return 0;
            }
            H = H->next;
            t = t->next;
        }
        return 1;
}


What I have tried:

I have tried by taking linked list as 3->NULL but it is showing runtime error . I am not getting where I have done the mistake . I checked whole code min of 3 times . please help me where I am going wrong.
Posted
Updated 6-Jun-21 21:44pm
Comments
CPallini 7-Jun-21 2:49am    
Some code is missing. You should post it in order to get better help.
merano99 7-Jun-21 19:43pm    
Wondering why there is old C Code in this C++ Program.
Would expect:

typedef std::deque<char> ListNode;

class Solution {
public:
Solution(std::string text);
bool lsPalindrome();
...
private:
ListNode _head, _reversed;
};
Patrice T 7-Jun-21 2:54am    
Which runtime error ?
Joe Woodbury 9-Jun-21 0:26am    
One issue is predecrementing n. If you have a list of 3 nodes. n is 1. Predecrementing it means you reverse the entire list. (And if you have a length of 1, the while loop runs twice whereupon temp is nullptr the second pass and it throws an exception.)

1 solution

Compiling does not mean your code is right! :laugh:
Think of the development process as writing an email: compiling successfully means that you wrote the email in the right language - English, rather than German for example - not that the email contained the message you wanted to send.

So now you enter the second stage of development (in reality it's the fourth or fifth, but you'll come to the earlier stages later): Testing and Debugging.

Start by looking at what it does do, and how that differs from what you wanted. This is important, because it give you information as to why it's doing it. For example, if a program is intended to let the user enter a number and it doubles it and prints the answer, then if the input / output was like this:
Input   Expected output    Actual output
  1            2                 1
  2            4                 4
  3            6                 9
  4            8                16
Then it's fairly obvious that the problem is with the bit which doubles it - it's not adding itself to itself, or multiplying it by 2, it's multiplying it by itself and returning the square of the input.
So with that, you can look at the code and it's obvious that it's somewhere here:
C
int Double(int value)
   {
   return value * value;
   }

Once you have an idea what might be going wrong, start using the debugger to find out why. Put a breakpoint on the first line of the method, and run your app. When it reaches the breakpoint, the debugger will stop, and hand control over to you. You can now run your code line-by-line (called "single stepping") and look at (or even change) variable contents as necessary (heck, you can even change the code and try again if you need to).
Think about what each line in the code should do before you execute it, and compare that to what it actually did when you use the "Step over" button to execute each line in turn. Did it do what you expect? If so, move on to the next line.
If not, why not? How does it differ?
Hopefully, that should help you locate which part of that code has a problem, and what the problem is.
This is a skill, and it's one which is well worth developing as it helps you in the real world as well as in development. And like all skills, it only improves by use!
 
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