C++ Linked List: Core Operations and Classic Problems

List Definition

struct ListNode {
    int val;
    ListNode* next;
    ListNode(int x) : val(x), next(nullptr) {}
};

Reverse Linked List

ListNode* reverseList(ListNode* head) {
    ListNode* prev = nullptr;
    ListNode* curr = head;

    while (curr) {
        ListNode* next = curr->next;
        curr->next = prev;
        prev = curr;
        curr = next;
    }

    return prev;
}

Detect Cycle (Floyd’s Algorithm)

bool hasCycle(ListNode* head) {
    ListNode* slow = head;
    ListNode* fast = head;

    while (fast && fast->next) {
        slow = slow->next;
        fast = fast->next->next;
        if (slow == fast) return true;
    }

    return false;
}

Merge Two Sorted Lists

ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
    ListNode dummy(0);
    ListNode* curr = &dummy;

    while (l1 && l2) {
        if (l1->val <= l2->val) {
            curr->next = l1;
            l1 = l1->next;
        } else {
            curr->next = l2;
            l2 = l2->next;
        }
        curr = curr->next;
    }

    curr->next = l1 ? l1 : l2;
    return dummy.next;
}

Remove Nth Node From End

ListNode* removeNthFromEnd(ListNode* head, int n) {
    ListNode dummy(0);
    dummy.next = head;
    ListNode* fast = &dummy;
    ListNode* slow = &dummy;

    for (int i = 0; i <= n; i++) fast = fast->next;

    while (fast) {
        fast = fast->next;
        slow = slow->next;
    }

    slow->next = slow->next->next;
    return dummy.next;
}

Practice Problems

• LeetCode 206: Reverse Linked List
• LeetCode 141: Linked List Cycle
• LeetCode 21: Merge Two Sorted Lists
• LeetCode 19: Remove Nth Node From End
• LeetCode 234: Palindrome Linked List
• LeetCode 148: Sort List