Find the smallest window in a string containing all characters of another string

Given two strings string1 and string2, find the smallest substring in string1 containing all characters of string2 efficiently.
For Example:

Input :  string = "this is a test string"
         pattern = "tist"
Output :  Minimum window is "t stri"
Explanation: "t stri" contains all the characters
              of pattern.

Input :  string = "geeksforgeeks"
         pattern = "ork" 
Output :  Minimum window is "ksfor"

Recommended: Please solve it on “PRACTICE ” first, before moving on to the solution.

Method 1 ( Brute force solution )
1- Generate all substrings of string1 (“this is a test string”)
2- For each substring, check whether the substring contains all characters of string2 (“tist”)
3- Finally, print the smallest substring containing all characters of string2.

Method 2 ( Efficient Solution )

1- First check if length of string is less than
   the length of given pattern, if yes
       then "no such window can exist ".
2- Store the occurrence of characters of given 
   pattern in a hash_pat[].
3- Start matching the characters of pattern with 
   the characters of string i.e. increment count 
   if a character matches
4- Check if (count == length of pattern ) this 
   means a window is found
5- If such window found, try to minimize it by 
   removing extra characters from beginning of
   current window.
6- Update min_length.
7- Print the minimum length window.

Diagram to explain above algorithm:
smallest-window

Below is program to implement above algorithm

C++

// C++ program to find smallest window containing 
// all characters of a pattern. 
#include<bits/stdc++.h> 
using namespace std; 
  
const int no_of_chars = 256; 
  
// Function to find smallest window containing 
// all characters of 'pat' 
string findSubString(string str, string pat) 
{ 
    int len1 = str.length(); 
    int len2 = pat.length(); 
  
    // check if string's length is less than pattern's 
    // length. If yes then no such window can exist 
    if (len1 < len2) 
    { 
        cout << "No such window exists"; 
        return ""; 
    } 
  
    int hash_pat[no_of_chars] = {0}; 
    int hash_str[no_of_chars] = {0}; 
  
    // store occurrence ofs characters of pattern 
    for (int i = 0; i < len2; i++) 
        hash_pat[pat[i]]++; 
  
    int start = 0, start_index = -1, min_len = INT_MAX; 
  
    // start traversing the string 
    int count = 0; // count of characters 
    for (int j = 0; j < len1 ; j++) 
    { 
        // count occurrence of characters of string 
        hash_str[str[j]]++; 
  
        // If string's char matches with pattern's char 
        // then increment count 
        if (hash_pat[str[j]] != 0 && 
            hash_str[str[j]] <= hash_pat[str[j]] ) 
            count++; 
  
        // if all the characters are matched 
        if (count == len2) 
        { 
            // Try to minimize the window i.e., check if 
            // any character is occurring more no. of times 
            // than its occurrence in pattern, if yes 
            // then remove it from starting and also remove 
            // the useless characters. 
            while ( hash_str[str[start]] > hash_pat[str[start]] 
                || hash_pat[str[start]] == 0) 
            { 
  
                if (hash_str[str[start]] > hash_pat[str[start]]) 
                    hash_str[str[start]]--; 
                start++; 
            } 
  
            // update window size 
            int len_window = j - start + 1; 
            if (min_len > len_window) 
            { 
                min_len = len_window; 
                start_index = start; 
            } 
        } 
    } 
  
    // If no window found 
    if (start_index == -1) 
    { 
    cout << "No such window exists"; 
    return ""; 
    } 
  
    // Return substring starting from start_index 
    // and length min_len 
    return str.substr(start_index, min_len); 
} 
  
// Driver code 
int main() 
{ 
    string str = "this is a test string"; 
    string pat = "tist"; 
  
    cout << "Smallest window is : \n"
        << findSubString(str, pat); 
    return 0; 
} 

Java

// Java program to find smallest window containing 
// all characters of a pattern. 
  
public class GFG  
{ 
    static final int no_of_chars = 256; 
      
    // Function to find smallest window containing 
    // all characters of 'pat' 
    static String findSubString(String str, String pat) 
    { 
        int len1 = str.length(); 
        int len2 = pat.length(); 
      
        // check if string's length is less than pattern's 
        // length. If yes then no such window can exist 
        if (len1 < len2) 
        { 
            System.out.println("No such window exists"); 
            return ""; 
        } 
      
        int hash_pat[] = new int[no_of_chars]; 
        int hash_str[] = new int[no_of_chars]; 
      
        // store occurrence ofs characters of pattern 
        for (int i = 0; i < len2; i++) 
            hash_pat[pat.charAt(i)]++; 
      
        int start = 0, start_index = -1, min_len = Integer.MAX_VALUE; 
      
        // start traversing the string 
        int count = 0; // count of characters 
        for (int j = 0; j < len1 ; j++) 
        { 
            // count occurrence of characters of string 
            hash_str[str.charAt(j)]++; 
      
            // If string's char matches with pattern's char 
            // then increment count 
            if (hash_pat[str.charAt(j)] != 0 && 
                hash_str[str.charAt(j)] <= hash_pat[str.charAt(j)] ) 
                count++; 
      
            // if all the characters are matched 
            if (count == len2) 
            { 
                // Try to minimize the window i.e., check if 
                // any character is occurring more no. of times 
                // than its occurrence in pattern, if yes 
                // then remove it from starting and also remove 
                // the useless characters. 
                while ( hash_str[str.charAt(start)] > hash_pat[str.charAt(start)] 
                    || hash_pat[str.charAt(start)] == 0) 
                { 
      
                    if (hash_str[str.charAt(start)] > hash_pat[str.charAt(start)]) 
                        hash_str[str.charAt(start)]--; 
                    start++; 
                } 
      
                // update window size 
                int len_window = j - start + 1; 
                if (min_len > len_window) 
                { 
                    min_len = len_window; 
                    start_index = start; 
                } 
            } 
        } 
      
        // If no window found 
        if (start_index == -1) 
        { 
        System.out.println("No such window exists"); 
        return ""; 
        } 
      
        // Return substring starting from start_index 
        // and length min_len 
        return str.substring(start_index, start_index + min_len); 
    } 
      
    // Driver Method 
    public static void main(String[] args) 
    { 
        String str = "this is a test string"; 
        String pat = "tist"; 
      
    System.out.print("Smallest window is :\n " + 
                        findSubString(str, pat)); 
    } 
} 

Output:

Smallest window is : 
t stri

This article is contributed by Sahil Chhabra (akku). If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

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