Strings from an array which are not prefix of any other string

Given an array arr[] of strings, the task is to print the strings from the array which are not prefix of any other string from the same array.

Examples:

Input: arr[] = {“apple”, “app”, “there”, “the”, “like”}
Output:
apple
like
there
Here “app” is a prefix of “apple”
Hence, it is not printed and
“the” is a prefix of “there”

Input: arr[] = {“a”, “aa”, “aaa”, “aaaa”}
Output:
aaaa

Naive approach: For every string of the array, we check if it is prefix of any other string. If it is then don’t display it.

Efficient approach: We pick strings from array one by one and insert it into Trie. Then there are two cases for the insertion of the string:

  1. While inserting if we found that the picked string is a prefix of an already inserted string then we don’t insert this string into the Trie.
  2. If a prefix is inserted first into the Trie and afterwards we find that the string is a prefix of some word then we simply make isEndOfWord = false for that particular node.

After constructing the Trie, we traverse it and display all the words in the Trie.

Below is the implementation of the above approach:

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// C++ implementation of the approach
#include <bits/stdc++.h>
using namespace std;
  
const int ALPHABET_SIZE = 26;
  
// Trie node
struct TrieNode {
    struct TrieNode* children[ALPHABET_SIZE];
  
    // isEndOfWord is true if the node represents
    // end of a word
    bool isEndOfWord;
};
  
// Returns new trie node (initialized to NULLs)
struct TrieNode* getNode(void)
{
    struct TrieNode* pNode = new TrieNode;
  
    pNode->isEndOfWord = false;
  
    for (int i = 0; i < ALPHABET_SIZE; i++)
        pNode->children[i] = NULL;
  
    return pNode;
}
  
// Function to insert a string into trie
void insert(struct TrieNode* root, string key)
{
    struct TrieNode* pCrawl = root;
  
    for (int i = 0; i < key.length(); i++) {
        int index = key[i] - 'a';
        if (!pCrawl->children[index])
            pCrawl->children[index] = getNode();
  
        // While inerting a word make
        // each isEndOfWord as false
        pCrawl->isEndOfWord = false;
        pCrawl = pCrawl->children[index];
    }
    int i;
  
    // Check if this word is prefix of
    // some already inserted word
    // If it is then don't insert this word
    for (i = 0; i < 26; i++) {
        if (pCrawl->children[i]) {
            break;
        }
    }
    // If present word is not prefix of
    // any other word then insert it
    if (i == 26) {
        pCrawl->isEndOfWord = true;
    }
}
  
// Function to display words in Trie
void display(struct TrieNode* root, char str[], int level)
{
    // If node is leaf node, it indicates end
    // of string, so a null charcter is added
    // and string is displayed
    if (root->isEndOfWord) {
        str[level] = '\0';
        cout << str << endl;
    }
  
    int i;
    for (i = 0; i < ALPHABET_SIZE; i++) {
  
        // If NON NULL child is found
        // add parent key to str and
        // call the display function recursively
        // for child node
        if (root->children[i]) {
            str[level] = i + 'a';
            display(root->children[i], str, level + 1);
        }
    }
}
  
// Driver code
int main()
{
    string keys[] = { "apple", "app", "there",
                      "the", "like" };
    int n = sizeof(keys) / sizeof(string);
  
    struct TrieNode* root = getNode();
  
    // Construct trie
    for (int i = 0; i < n; i++)
        insert(root, keys[i]);
  
    char str[100];
    display(root, str, 0);
  
    return 0;
}

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Output:

apple
like
there


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