Sorting array of strings (or words) using Trie

Given an array of strings, print them in alphabetical (dictionary) order. If there are duplicates in input array, we need to print them only once.


Input : "abc", "xy", "bcd"
Output : abc bcd xy         

Input : "geeks", "for", "geeks", "a", "portal", 
        "to", "learn", "can", "be", "computer", 
        "science", "zoom", "yup", "fire", "in", "data"
Output : a be can computer data fire for geeks
         in learn portal science to yup zoom

Trie is an efficient data structure used for storing data like strings. To print the string in alphabetical order we have to first insert in the trie and then perform preorder traversal to print in alphabetical order.

// C++ program to sort an array of strings
// using Trie
#include <bits/stdc++.h>
using namespace std;

const int MAX_CHAR = 26;

struct Trie {

    // index is set when node is a leaf
    // node, otherwise -1;
    int index;

    Trie* child[MAX_CHAR];

    /*to make new trie*/
        for (int i = 0; i < MAX_CHAR; i++)
            child[i] = NULL;
        index = -1;

/* function to insert in trie */
void insert(Trie* root, string str, int index)
    Trie* node = root;

    for (int i = 0; i < str.size(); i++) {

        /* taking ascii value to find index of
          child node */
        char ind = str[i] - 'a';

        /* making new path if not already */
        if (!node->child[ind])
            node->child[ind] = new Trie();

        // go to next node
        node = node->child[ind];

    // Mark leaf (end of word) and store
    // index of word in arr[]
    node->index = index;

/* function for preorder traversal */
bool preorder(Trie* node, string arr[])
    if (node == NULL)
        return false;

    for (int i = 0; i < MAX_CHAR; i++) {
        if (node->child[i] != NULL) {

            /* if leaf node then print key*/
            if (node->child[i]->index != -1)
                cout << arr[node->child[i]->index] << endl;

            preorder(node->child[i], arr);

void printSorted(string arr[], int n)
    Trie* root = new Trie();

    // insert all keys of dictionary into trie
    for (int i = 0; i < n; i++)
        insert(root, arr[i], i);

    // print keys in lexicographic order
    preorder(root, arr);

// Driver code
int main()
    string arr[] = { "abc", "xy", "bcd" };
    int n = sizeof(arr) / sizeof(arr[0]);
    printSorted(arr, n);
    return 0;


abc bcd xy   

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