LeetCode/problems/palindrome-linked-list.cpp

#include <cassert>
#include <vector>

/**
 * 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) {}
 * };
 */

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)
    {
    }
};

namespace {

struct middle_t {
    ListNode* node;
    std::size_t size;

    middle_t(ListNode* node, std::size_t size)
        : node(node)
        , size(size)
    {
    }
};

middle_t
find_middle(ListNode* head)
{
    ListNode* slow = head;
    ListNode* fast = head->next;

    std::size_t size;
    for (size = 0; fast != nullptr && fast->next != nullptr; size++) {
        slow = slow->next;
        fast = fast->next->next;
    }

    return { slow, size };
}

ListNode* reverse_list(ListNode* tail)
{
    ListNode* previous = nullptr;
    ListNode* next = nullptr;

    for (ListNode* current = tail; current != nullptr; previous = current, current = next) {
        next = current->next;
        current->next = previous;
    }

    return previous;
}

bool compare_lists(ListNode* left, ListNode* right, std::size_t size)
{
    for (auto i = 0u; i < size + 1; i++) {
        if (left->val != right->val) {
            return false;
        }
        left = left->next;
        right = right->next;
    }

    return true;
}

} // namespace

class Solution {
public:
    bool isPalindrome(ListNode* head)
    {
        // find middle of the linked list
        auto mid = find_middle(head);

        // reverse the tail of the linked list
        auto tail = reverse_list(mid.node);

        // compare head and tail
        return compare_lists(head, tail, mid.size);
    }
};

#pragma region Testing utilities
namespace {

ListNode*
construct_list(const std::vector<int>& values)
{
    ListNode* head = new ListNode(values.front());
    ListNode* tail = head;

    for (std::size_t i = 1; i < values.size(); i++) {
        tail->next = new ListNode(values[i]);
        tail = tail->next;
    }

    return head;
}

void destroy_list(ListNode* linked_list)
{
    if (linked_list == nullptr) {
        return;
    }
    destroy_list(linked_list->next);
    delete linked_list;
}

bool test_find_middle(const std::vector<int>& values, int mid_value)
{
    auto linked_list = construct_list(values);

    auto mid = find_middle(linked_list);
    auto result = mid.node->val == mid_value;

    destroy_list(linked_list);
    return result;
}

bool test_isPalindrome(const std::vector<int>& values)
{
    auto linked_list = construct_list(values);

    Solution s;
    auto result = s.isPalindrome(linked_list);

    destroy_list(linked_list);
    return result;
}

} // namespace
#pragma endregion // Testing utilities

int main()
{
    // find_middle tests
    assert(test_find_middle(std::vector { 1, 2, 2, 1 }, 2));
    assert(test_find_middle(std::vector { 1, 2, 3, 2, 1 }, 3));
    assert(test_find_middle(std::vector { 1, 2 }, 1));
    assert(test_find_middle(std::vector { 1 }, 1));

    // isPalindrome tests
    assert(test_isPalindrome(std::vector { 1, 2, 2, 1 }));
    assert(test_isPalindrome(std::vector { 1, 2, 3, 2, 1 }));
    assert(!test_isPalindrome(std::vector { 1, 2 }));
    assert(test_isPalindrome(std::vector { 1 }));

    return 0;
}