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Sort an array of strings based on the frequency of good words in them
  • Difficulty Level : Hard
  • Last Updated : 10 Jul, 2019

Given a set of product reviews (R) by different customers and a string S containing good words separated by a _, the task is to sort the reviews in decreasing order of their goodness value.
Goodness Value is defined by the number of good words present in that review.


Input: S = “geeks_for_geeks_is_great”,
R = {“geeks_are_geeks”, “geeks_dont_lose”, “geeks_for_geeks_is_love”}
geeks for geeks is love
geeks are geeks
geeks dont lose

Input: S = “cool_wifi_ice”,
R = {“water_is_cool”, “cold_ice_drink”, “cool_wifi_speed”}
cool wifi speed
water is cool
cold ice drink

Naive approach: Insert all the good words in an unordered_set and then iterate through each word of each sentence of the reviews array and keep a count of the good words by checking if this word is present in that set of good words. We then use a stable sorting algorithm and sort the array R according to the count of good words in each review present in R. It’s clear that the time complexity of this method is greater than O(N * NlogN) .

Efficient approach: Make a Trie of all the good words and check the goodness of each word in a review using the trie.

  1. Insert all the good words in a trie.
  2. For each review, calculate the number of good words in it by checking whether a given word exists in the trie or not.

Below is the implementation of the above approach:

// C++ implementation of the approach
#include <bits/stdc++.h>
using namespace std;
#define F first
#define S second
#define MAX 26
// Comparator function for sorting
bool cmp(const pair<int, int>& a, const pair<int, int>& b)
    // Compare the number of good words
    if (a.F == b.F)
        return a.S < b.S;
    return a.F > b.F;
// Structure of s Trie node
struct node {
    bool exist;
    node* arr[MAX];
    node(bool bul = false)
        exist = bul;
        for (int i = 0; i < MAX; i++)
            arr[i] = NULL;
// Function to add a string to the trie
void add(string s, node* trie)
    // Add a node to the trie
    int n = s.size();
    for (int i = 0; i < n; i++) {
        // If trie doesn't already contain
        // the current node then create one
        if (trie->arr[s[i] - 'a'] == NULL)
            trie->arr[s[i] - 'a'] = new node();
        trie = trie->arr[s[i] - 'a'];
    trie->exist = true;
// Function that returns true if
// the trie contains the string s
bool search(string s, node* trie)
    // Search for a node in the trie
    for (int i = 0; i < s.size(); i++) {
        if (trie->arr[s[i] - 'a'] == NULL)
            return false;
        trie = trie->arr[s[i] - 'a'];
    return trie->exist;
// Function to replace every '_' with a
// white space in the given string
void convert(string& str)
    // Convert '_' to spaces
    for (int i = 0; i < str.size(); i++)
        if (str[i] == '_')
            str[i] = ' ';
// Function to sort the array based on good words
void sortArr(string good, vector<string>& review)
    // Extract all the good words which
    // are '_' separated
    node* trie = new node();
    string word;
    stringstream ss;
    ss << good;
    // Building the entire trie by stringstreaming
    // the 'good words' string
    while (ss >> word)
        add(word, trie);
    int k, n = review.size();
    // To store the number of good words
    // and the string index pairs
    vector<pair<int, int> > rating(n);
    for (int i = 0; i < n; i++) {
        ss << review[i];
        k = 0;
        while (ss >> word) {
            // If this word is present in the trie
            // then increment its count
            if (search(word, trie))
        // Store the number of good words in the
        // current string paired with its
        // index in the original array
        rating[i].F = k;
        rating[i].S = i;
    // Using comparator function to
    // sort the array as required
    sort(rating.begin(), rating.end(), cmp);
    // Print the sorted array
    for (int i = 0; i < n; i++)
        cout << review[rating[i].S] << "\n";
// Driver code
int main()
    // String containing good words
    string S = "geeks_for_geeks_is_great";
    // Vector of strings to be sorted
    vector<string> R = { "geeks_are_geeks", "geeks_dont_lose",
                         "geeks_for_geeks_is_love" };
    // Sort the array based on the given conditions
    sortArr(S, R);
geeks for geeks is love
geeks are geeks
geeks dont lose

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