nthToLastNode.cpp

/*
 * Find nth to last node in a linked list.
 *
 */



#include<iostream>


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

ListNode* nthToLastNode( ListNode *head, int n )
{
    ListNode *ptr1 = head;
    ListNode *ptr2 = head;

    int i = 1;
    while ( ptr1 && i <= n) {
        ptr1 = ptr1->next;
        ++i;
    }
    while( ptr1) {
        ptr1 = ptr1->next;
        ptr2 = ptr2->next;
    }
    return ptr2;
}

void insert( ListNode* & head, int v )
{
    ListNode *newNode = new ListNode(v);
    if ( head == nullptr) {
        head = newNode;
    } else {
        ListNode *temp = head;
        while( temp->next != nullptr ) {
            temp = temp->next;
        }
        temp->next = newNode;
    }
}

void iterateList(ListNode *head) {
    while(head) {
        std::cout << head->val << "-->";
        head = head->next;
    }
    std::cout << "NULL" <<  std::endl;
}

int main()
{
    ListNode * head = nullptr;
    insert(head, 3);
    insert(head, 2);
    insert(head, 1);
    insert(head, 5);
    iterateList(head);

    ListNode *nthNodeToLastNode = nthToLastNode(head, 2);
    std::cout << nthNodeToLastNode->val << std::endl;

}