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Find last unique URL from long list of URLs in single traversal
  • Difficulty Level : Hard
  • Last Updated : 30 Nov, 2016

Given a very long list of URLs, find out last unique URL. Only one traversal of all URLs is allowed.




We can solve this problem in one traversal by using Trie with a Doubly Linked List (We can insert and delete in O(1) time). The idea is to insert all URLs into the Trie one by one and check if it is duplicate or not. To know if we have previously encountered the URL, we need to mark the last node of every URL as leaf node. If we encounter a URL for the first time, we insert it into the doubly Linked list and maintain a pointer to that node in linked list in leaf node of the trie. If we encounter a URL that is already in the trie and has pointer to the url in the linked list, we delete the node from the linked list and set its pointer in the trie to null. After all URLs are processed, linked list will only contain the URLs that are distinct and the node at the beginning of the linked list will be last unique URL.

// C++ program to print distinct URLs using Trie
// and Doubly Linked List
#include <bits/stdc++.h>
using namespace std;
// Alphabet size (# of symbols)
const int ALPHABET_SIZE = 256;
// A linked list node
struct DLLNode
    string data;
    DLLNode* next, * prev;
// trie node
struct TrieNode
    TrieNode* children[ALPHABET_SIZE];
    // isLeaf is true if the node represents
    // end of a word
    bool isLeaf;
    DLLNode* LLptr;
/* Given a reference (pointer to pointer) to the
   head of a list and an int, inserts a new node
   on the front of the list. */
void push(DLLNode*& head_ref, string new_data)
    DLLNode* new_node = new DLLNode;
    // put in the data
    new_node->data = new_data;
    // Make next of new node as head and previous
    // as NULL
    new_node->next = (head_ref);
    new_node->prev = NULL;
    // change prev of head node to new node
    if(head_ref != NULL)
        head_ref->prev = new_node;
    // move the head to point to the new node
    head_ref = new_node;
/* Function to delete a node in a Doubly Linked List.
   head_ref --> pointer to head node pointer.
   del  -->  pointer to node to be deleted. */
void deleteNode(DLLNode*& head_ref, DLLNode* del)
    // base case
    if (head_ref == NULL || del == NULL)
    // If node to be deleted is head node
    if (head_ref == del)
        head_ref = del->next;
    // Change next only if node to be deleted is
    // NOT the last node
    if (del->next != NULL)
        del->next->prev = del->prev;
    // Change prev only if node to be deleted is
    // NOT the first node
    if (del->prev != NULL)
        del->prev->next = del->next;
    // Finally, free the memory occupied by del
// Returns new trie node (initialized to NULLs)
TrieNode* getNewTrieNode(void)
    TrieNode* pNode = new TrieNode;
    if (pNode)
        pNode->isLeaf = false;
        for (int i = 0; i < ALPHABET_SIZE; i++)
            pNode->children[i] = NULL;
        pNode->LLptr = NULL;
    return pNode;
// If not present, inserts key into trie
// If the key is prefix of trie node, just marks leaf node
void insert(TrieNode* root, string key, DLLNode*& head)
    int index;
    TrieNode* pCrawl = root;
    for (int level = 0; level < key.length(); level++)
        index = int(key[level]);
        if (!pCrawl->children[index])
            pCrawl->children[index] = getNewTrieNode();
        pCrawl = pCrawl->children[index];
    if (pCrawl->isLeaf)
        // cout << "Duplicate Found " << key << endl;
        // delete from linked list
        if (pCrawl->LLptr)
            deleteNode(head, pCrawl->LLptr);
        pCrawl->LLptr = NULL;
        // mark last node as leaf
        pCrawl->isLeaf = true;
        // insert to linked list
        push(head, key);
        pCrawl->LLptr = head;
// Driver function
int main()
    string urls[] = {
    TrieNode* root = getNewTrieNode();
    // Start with the empty list
    DLLNode* head = NULL;
    int n = sizeof(urls)/sizeof(urls[0]);
    // Construct Trie from given URLs
    for (int i = 0; i < n; i++)
        insert(root, urls[i], head);
    // head of linked list will point to last
    // distinct URL
    cout << head->data << endl;
    return 0;


This article is contributed by Aditya Goel. If you like GeeksforGeeks and would like to contribute, you can also write an article using or mail your article to See your article appearing on the GeeksforGeeks main page and help other Geeks.

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