Sort an array of strings based on the given order

Difficulty Level : Expert
Last Updated : 07 May, 2020

Given an array of strings words[] and the sequential order of alphabets, our task is to sort the array according to the order given. Assume that the dictionary and the words only contain lowercase alphabets.

Examples:

Input: words = {“hello”, “geeksforgeeks”}, order = “hlabcdefgijkmnopqrstuvwxyz”
Output: “hello”, “geeksforgeeks”
Explanation:
According to the given order ‘h’ occurs before ‘g’ and hence the words are considered to be sorted.

Input: words = {“word”, “world”, “row”}, order = “worldabcefghijkmnpqstuvxyz”
Output: “world”, “word”, “row”
Explanation:
According to the given order ‘l’ occurs before ‘d’ hence the words “world” will be kept first.

Approach: To solve the problem mentioned above we need to maintain the priority of each character in the given order. For doing that use Map Data Structure.

Iterate in the given order and set the priority of a character to its index value.
Use a custom comparator function to sort the array.
In the comparator function, iterate through the minimum sized word between the two words and try to find the first different character, the word with the lesser priority value character will be the smaller word.
If the words have the same prefix, then the word with a smaller size is the smaller word.

Below is the implementation of above approach:

// C++ program to sort an array 
// of strings based on the given order 
  
#include <bits/stdc++.h> 
using namespace std; 
  
// For storing priority of each character 
unordered_map<char, int> mp; 
  
// Custom comparator function for sort 
bool comp(string& a, string& b) 
{ 
  
    // Loop through the minimum size 
    // between two words 
    for (int i = 0; 
         i < min(a.size(), b.size()); 
         i++) { 
  
        // Check if the characters 
        // at position i are different, 
        // then the word containing lower 
        // valued character is smaller 
        if (mp[a[i]] != mp[b[i]]) 
            return mp[a[i]] < mp[b[i]]; 
    } 
  
    /* When loop breaks without returning, it 
       means the prefix of both words were same 
       till the execution of the loop. 
       Now, the word with the smaller size will 
       occur before in sorted order 
        */
    return (a.size() < b.size()); 
} 
  
// Function to print the 
// new sorted array of strings 
void printSorted(vector<string> words, 
                 string order) 
{ 
  
    // Mapping each character 
    // to its occurance position 
    for (int i = 0; i < order.size(); i++) 
        mp[order[i]] = i; 
  
    // Sorting with custom sort function 
    sort(words.begin(), words.end(), comp); 
  
    // Printing the sorted order of words 
    for (auto x : words) 
        cout << x << " "; 
} 
  
// Driver code 
int main() 
{ 
  
    vector<string> words 
        = { "word", "world", "row" }; 
    string order 
        = { "worldabcefghijkmnpqstuvxyz" }; 
  
    printSorted(words, order); 
  
    return 0; 
} 

Output:

world word row

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