Question

Codelab question!

This is c++.

Consider the following code: // Linked Lists: TRAVERSE

struct ListNode { int data; struct ListNode *next; };

Assume that a linked list has been created and head points to a sentinel node. A sentinel node is an empty data node in the beginning of the list. It sometimes holds a sentinel value. The use of sentinel nodes is a popular trick to simplify the insert and delete operations.

You may also assume that the list is not empty. Each node in the list contains an integer representing a digit in a number. The data in the sentinel node is -1.

For instance, n = 234 will be stored as {-1, 4, 3, 2} in the linked list.

Write a function based on the list traversal named computeNumFromList that is passed the pointer to the first node in the list, it prints the digits in the list one per line, and it returns a long integer, the number calculated from its digits in the list (234 see the above example). You may assume that long int will hold the converted number (no need to check for numeric overflow).

This is what I got :

long int computeNumFromList(ListNode *root)
{
long int ans = 0;
root = root->next;
long int ten=1;
while(root!=nullptr)
{
ans = ans + ten*root->data;
root = root->next;
ten = ten*10;
}
return ans;
}

Some how codelab still said that I am incorrect. Could someone please help me? :(

Answer 1

From the problem statement:

"..... named computeNumFromList that is passed the pointer to the first node in the list, it prints the digits in the list one per line, and it returns a long integer, .... "

You missed the print statement. Also, it is not clear whether this function is meant to handle negative numbers or not. In case it needs to handle negative numbers, you can add another node containing -1 (besides the sentinel) while creating the list to make it independent, otherwise you would have to check the original number and convert the returned result from the function.

-------------------- Code ------------------------

// This code handles negative numbers as well

#include <iostream>

struct ListNode
{
    int data;
    struct ListNode *next;
    ListNode() : data{0}, next{nullptr}
    {}
};

void print_list(ListNode *root)
{
    while(root != nullptr)
    {
        std::cout << root->data << "\n";
        root = root->next;
    }
}
void process_number(long int number, ListNode *root)
{
    if(root != nullptr)
    {
        if(number < 0)
        {
            root->next = new ListNode;
            root = root->next;
            root->data = -1;
            number = -number;
        }
        while(number > 0)
        {
            root->next = new ListNode;
            root = root->next;
            root->data = number % 10;
            number = number / 10;
        }
    }
}

long int computeNumFromList(ListNode *root)
{
    long int ans = 0;
    bool neg = false;
    root = root->next;
    long int ten=1;
    while(root!=nullptr)
    {
        if(root->data == -1)
        {
            neg = true;
            root = root->next;
        }
        std::cout << root->data << "\n";
        ans = ans + ten*root->data;
        root = root->next;
        ten = ten*10;
    }
    return neg ? -ans : ans;
}

int main()
{
    int number;
    std::cin >> number;
    ListNode *root = new ListNode;
    root->data = -1;
    root->next = nullptr;
    process_number(number, root);
    std::cout << computeNumFromList(root) << "\n";
}

---------------- Output ----------------