Print Strings In Reverse Dictionary Order Using Trie

Trie is an efficient information retrieval data structure. Using Trie, search complexities can be brought to an optimal limit. 
Given an array of strings. The task is to print all strings in reverse dictionary order using Trie. If there are duplicates in the input array, we need to print them only once.

Examples:

Input: str = {"cat", "there", "caller", "their", "calling"}
Output: there
        their
        cat
        calling
        caller

                                       root
                                      /     \    
                                     c       t     
                                     |       |     
                                     a       h     
                                     | \     |     
                                     l  t    e      
                                     |       |  \
                                     l       i   r
                                     | \     |   |
                                     e  i    r   e
                                     |  |
                                     r  n
                                        |
                                        g 

Input: str = {"Candy", "cat", "Caller", "calling"}
Output: cat
        candy
        calling
        caller
                                        root
                                         |    
                                         c            
                                         |            
                                         a            
                                      /   | \        
                                     l    n  t       
                                     |    |       
                                     l    d       
                                     | \  |      
                                     e  i y      
                                     |  |
                                     r  n
                                        |
                                        g

Approach:

To solve the problem mentioned above, first, construct a Trie using all strings then print a string of rightmost subtree from top to bottom then print a string of second right subtree from top to bottom then print for third right subtree and so on. It is similar to preorder traversal of a tree from right to left.

Below is the implementation of the above approach:  

C++

// C++ program to print array of string
// in reverse dictionary order using trie

#include <bits/stdc++.h>
using namespace std;

#define CHILDREN 26
#define MAX 100

// Trie node
struct trie {
    trie* child[CHILDREN];

    // endOfWord is true
    // if the node represents
    // end of a word
    bool endOfWord;
};

// Function will return
// the new node initialized NULL
trie* createNode()
{
    trie* temp = new trie();

    temp->endOfWord = false;

    for (int i = 0; i < CHILDREN; i++) {

        // Initialize null to the all child
        temp->child[i] = NULL;
    }
    return temp;
}

// Function will insert the
// string in a trie recursively
void insertRecursively(trie* itr,
                       string str, int i)
{

    if (i < str.length()) {
        int index = str[i] - 'a';

        if (itr->child[index] == NULL) {

            // Create a new node
            itr->child[index] = createNode();
        }

        // Recursive call for insertion of string
        insertRecursively(itr->child[index], str, i + 1);
    }
    else {

        // Make the endOfWord
        // true which represents
        // the end of string
        itr->endOfWord = true;
    }
}

// Function call to insert a string
void insert(trie* itr, string str)
{
    // Function call with necessary arguments
    insertRecursively(itr, str, 0);
}

// Function to check whether the node is leaf or not
bool isLeafNode(trie* root)
{
    return root->endOfWord != false;
}

// Function to display the content of trie
void displayContent(trie* root, char str[], int level)
{

    // If node is leaf node, it indicates end
    // of string, so a null character is added
    // and string is displayed
    if (isLeafNode(root)) {
        // Assign a null character in temporary string
        str[level] = '\0';
        cout << str << endl;
    }

    for (int i = CHILDREN - 1; i >= 0; i--) {

        // check if NON NULL child is found
        // add parent key to str and
        // call the display function recursively
        // for child node
        if (root->child[i]) {
            str[level] = i + 'a';
            displayContent(root->child[i], str, level + 1);
        }
    }
}

// Function call for displaying content
void display(trie* itr)
{
    int level = 0;

    char str[MAX];

    displayContent(itr, str, level);
}

// Driver code
int main()
{
    trie* root = createNode();
    insert(root, "their");
    insert(root, "there");
    insert(root, "answer");
    insert(root, "any");

    /* After inserting strings, trie will look like
                                        root
                                        / \
                                        a     t
                                        |     |
                                        n     h
                                        | \ |
                                        s y e
                                        |     | \
                                        w     i r
                                        |     | |
                                        e     r e
                                        |
                                        r
    */

    display(root);

    return 0;
}

Python3

# Python3 program to print array of string
# in reverse dictionary order using trie

CHILDREN = 26
MAX = 100

# Trie node
class trie:

    def __init__(self):
        self.child = [0 for i in range(CHILDREN)]

        # endOfWord is true
        # if the node represents
        # end of a word
        self.endOfWord = False;

# Function will return
# the new node initialized NONE
def createNode():

    temp = trie();

    temp.endOfWord = False;

    for i in range(CHILDREN):

        # Initialize null to the all child
        temp.child[i] = None;

    return temp;

# Function will insert the
# string in a trie recursively
def insertRecursively(itr, str, i):

    if (i < len(str)):

        index = ord(str[i]) - ord('a');

        if (itr.child[index] == None):

            # Create a new node
            itr.child[index] = createNode();

        # Recursive call for insertion of string
        insertRecursively(itr.child[index], str, i + 1);

    else:

        # Make the endOfWord
        # true which represents
        # the end of string
        itr.endOfWord = True;

# Function call to insert a string
def insert(itr, str):

    # Function call with necessary arguments
    insertRecursively(itr, str, 0);

# Function to check whether the node is leaf or not
def isLeafNode(root):

    return root.endOfWord != False;

# Function to display the content of trie
def displayContent(root, str, level):

    # If node is leaf node, it indicates end
    # of string, so a null character is added
    # and string is displayed
    if (isLeafNode(root)):

        # Assign a null character in temporary string
        print("".join(str[:level]))

    for i in range(CHILDREN-1, -1, -1):

        # check if NON NONE child is found
        # add parent key to str and
        # call the display function recursively
        # for child node
        if (root.child[i]):
            str[level] = chr(i + ord('a'));
            displayContent(root.child[i], str, level + 1);

# Function call for displaying content
def display(itr):

    level = 0;

    str = ['' for i in range(MAX)];

    displayContent(itr, str, level);

# Driver code
if __name__=='__main__':

    root = createNode();

    insert(root, "their");
    insert(root, "there");
    insert(root, "answer");
    insert(root, "any");

    ''' After inserting strings, trie will look like
                                        root
                                        / \
                                        a     t
                                        |     |
                                        n     h
                                        | \ |
                                        s y e
                                        |     | \
                                        w     i r
                                        |     | |
                                        e     r e
                                        |
                                        r
    '''

    display(root);

# This code is contributed by rutvik_56

Output:

there
their
any
answer