Bottom-up traversal of a Trie

Trie is an efficient information retrieval data structure. Using Trie, search complexities can be brought to an optimal limit (key length).

Given a trie. The task is to print the characters in a bottom-up manner

Bottum-up traversal:



First print string of left most subtree(from bottom to top) then print string of second left subtree(from bottom to top) then print for third left subtree and so on.

It is similar to post-order traversal of a tree

Example:

    
Input :
                                          root
                                         /    \    
                                         a     t     
                                         |     |     
                                         n     h     
                                         | \   |   
                                         s  y  e     
                                         |     |  \ 
                                         w     i   r
                                         |     |   |
                                         e     r   e
                                         |        
                                         r        
Output : r, e, w, s, y, n, a, r, i, e, r, e, h, t  

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

Output : r, e, g, n, i, l, l, t, a, c, r, i, e, r, e, h, t

Explanation :
In the first example, the root has two parts. First part contains strings: “answer” and “any”.
the second part with strings “their” and “there”.

  • Now first we got to left subtree containing strings “answer” and “any” which separates by character ‘n’. Now ‘n’ separates two-part of characters ‘s’, ‘w’, ‘e’, ‘r’ and ‘y’. so print ‘s’, ‘w’, ‘e’, ‘r’ in reverse order then print ‘y’ and go up and print ‘n'(which separates string) then go up and print ‘a’. Now first left subtree has printed in bottom up manner ‘r’, ‘e’, ‘w’, ‘s’, ‘y’, ‘n’, ‘a’.
  • Do the same thing for another subtree of the root which contains strings “their” and “there” which is separated by character ‘e’.

Approach :
The idea to do this is to start traversing from the root node of the trie, whenever we find a NON-NULL child node, we recursively move ahead when we get “NULL” we return simply and print the value of current node and same goes un till we find the node which is a leaf node, which actually marks the end of the string.

Below is the implementation of the above approach :

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// CPP program to traverse in bottom up manner
#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 to NULLs)
trie* createNode()
{
    trie* temp = new trie();
    temp->endOfWord = false;
    for (int i = 0; i < CHILDREN; i++) {
        // Initialise all child to the null, initially
        temp->child[i] = NULL;
    }
      
    // Return newly created node
    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) {
            // Assining a newly created node
            itr->child[index] = createNode();
        }
        // Recursive call for insertion 
        // of a string in a trie
        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 print traverse 
// the tree in bottum to top manner
void printPattern(trie* itr)
{
    // Base condition
    if (itr == NULL)
        return;
  
    // Loop for printing t a value of character
    for (int i = 0; i < CHILDREN; i++) {
          
        // Recursive call for print pattern
        printPattern(itr->child[i]);
        if (itr->child[i] != NULL) {
            char ch = (i + 97);
            cout << ch << ", "; // Print character
        }
    }
}
  
// Driver code
int main()
{
    trie* root = createNode();
      
    // Function to insert a string
    insert(root, "thier"); 
    insert(root, "there");
    insert(root, "answer");
    insert(root, "any");
      
      
    // Function call for printing a pattern
    printPattern(root);
  
    return 0;
}

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Output:

 r, e, w, s, y, n, a, e, r, e, r, e, i, h, t, 


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