Anagram Substring Search (Or Search for all permutations)

• Difficulty Level : Medium
• Last Updated : 07 Jul, 2021

Given a text txt[0..n-1] and a pattern pat[0..m-1], write a function search(char pat[], char txt[]) that prints all occurrences of pat[] and its permutations (or anagrams) in txt[]. You may assume that n > m.
Expected time complexity is O(n)
Examples:

1) Input:  txt[] = "BACDGABCDA"  pat[] = "ABCD"
Output:   Found at Index 0
Found at Index 5
Found at Index 6
2) Input: txt[] =  "AAABABAA" pat[] = "AABA"
Output:   Found at Index 0
Found at Index 1
Found at Index 4

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This problem is slightly different from standard pattern searching problem, here we need to search for anagrams as well. Therefore, we cannot directly apply standard pattern searching algorithms like KMP, Rabin Karp, Boyer Moore, etc.
A simple idea is to modify Rabin Karp Algorithm. For example, we can keep the hash value as sum of ASCII values of all characters under modulo of a big prime number. For every character of text, we can add the current character to hash value and subtract the first character of previous window. This solution looks good, but like standard Rabin Karp, the worst case time complexity of this solution is O(mn). The worst case occurs when all hash values match and we one by one match all characters.
We can achieve O(n) time complexity under the assumption that alphabet size is fixed which is typically true as we have maximum 256 possible characters in ASCII. The idea is to use two count arrays:
1) The first count array store frequencies of characters in pattern.
2) The second count array stores frequencies of characters in current window of text.
The important thing to note is, time complexity to compare two count arrays is O(1) as the number of elements in them are fixed (independent of pattern and text sizes). Following are steps of this algorithm.
1) Store counts of frequencies of pattern in first count array countP[]. Also store counts of frequencies of characters in first window of text in array countTW[].
2) Now run a loop from i = M to N-1. Do following in loop.
…..a) If the two count arrays are identical, we found an occurrence.
…..b) Increment count of current character of text in countTW[]
…..c) Decrement count of first character in previous window in countWT[]
3) The last window is not checked by above loop, so explicitly check it.
Following is the implementation of above algorithm.

C++

 // C++ program to search all anagrams of a pattern in a text#include#include#define MAX 256using namespace std; // This function returns true if contents of arr1[] and arr2[]// are same, otherwise false.bool compare(char arr1[], char arr2[]){    for (int i=0; i

Java

 // Java program to search all anagrams// of a pattern in a textpublic class GFG{    static final int MAX = 256;         // This function returns true if contents    // of arr1[] and arr2[] are same, otherwise    // false.    static boolean compare(char arr1[], char arr2[])    {        for (int i = 0; i < MAX; i++)            if (arr1[i] != arr2[i])                return false;        return true;    }     // This function search for all permutations    // of pat[] in txt[]    static void search(String pat, String txt)    {        int M = pat.length();        int N = txt.length();         // countP[]:  Store count of all        // characters of pattern        // countTW[]: Store count of current        // window of text        char[] countP = new char[MAX];        char[] countTW = new char[MAX];        for (int i = 0; i < M; i++)        {            (countP[pat.charAt(i)])++;            (countTW[txt.charAt(i)])++;        }         // Traverse through remaining characters        // of pattern        for (int i = M; i < N; i++)        {            // Compare counts of current window            // of text with counts of pattern[]            if (compare(countP, countTW))                System.out.println("Found at Index " +                                          (i - M));                         // Add current character to current            // window            (countTW[txt.charAt(i)])++;             // Remove the first character of previous            // window            countTW[txt.charAt(i-M)]--;        }         // Check for the last window in text        if (compare(countP, countTW))            System.out.println("Found at Index " +                                       (N - M));    }     /* Driver program to test above function */    public static void main(String args[])    {        String txt = "BACDGABCDA";        String pat = "ABCD";        search(pat, txt);    }}// This code is contributed by Sumit Ghosh

Python3

 # Python program to search all# anagrams of a pattern in a text MAX=256 # This function returns true# if contents of arr1[] and arr2[]# are same, otherwise false.def compare(arr1, arr2):    for i in range(MAX):        if arr1[i] != arr2[i]:            return False    return True     # This function search for all# permutations of pat[] in txt[] def search(pat, txt):     M = len(pat)    N = len(txt)     # countP[]:  Store count of    # all characters of pattern    # countTW[]: Store count of    # current window of text    countP = *MAX     countTW = *MAX     for i in range(M):        (countP[ord(pat[i]) ]) += 1        (countTW[ord(txt[i]) ]) += 1     # Traverse through remaining    # characters of pattern    for i in range(M,N):         # Compare counts of current        # window of text with        # counts of pattern[]        if compare(countP, countTW):            print("Found at Index", (i-M))         # Add current character to current window        (countTW[ ord(txt[i]) ]) += 1         # Remove the first character of previous window        (countTW[ ord(txt[i-M]) ]) -= 1         # Check for the last window in text       if compare(countP, countTW):        print("Found at Index", N-M)         # Driver program to test above function      txt = "BACDGABCDA"pat = "ABCD"      search(pat, txt)   # This code is contributed# by Upendra Singh Bartwal

C#

 // C# program to search all anagrams// of a pattern in a textusing System; class GFG{public const int MAX = 256; // This function returns true if // contents of arr1[] and arr2[]// are same, otherwise false.public static bool compare(char[] arr1,                           char[] arr2){    for (int i = 0; i < MAX; i++)    {        if (arr1[i] != arr2[i])        {            return false;        }    }    return true;} // This function search for all// permutations of pat[] in txt[]public static void search(string pat,                          string txt){    int M = pat.Length;    int N = txt.Length;     // countP[]: Store count of all    // characters of pattern    // countTW[]: Store count of current    // window of text    char[] countP = new char[MAX];    char[] countTW = new char[MAX];    for (int i = 0; i < M; i++)    {        (countP[pat[i]])++;        (countTW[txt[i]])++;    }     // Traverse through remaining    // characters of pattern    for (int i = M; i < N; i++)    {        // Compare counts of current window        // of text with counts of pattern[]        if (compare(countP, countTW))        {            Console.WriteLine("Found at Index " +                             (i - M));        }         // Add current character to        // current window        (countTW[txt[i]])++;         // Remove the first character of        // previous window        countTW[txt[i - M]]--;    }     // Check for the last window in text    if (compare(countP, countTW))    {        Console.WriteLine("Found at Index " +                         (N - M));    }} // Driver Codepublic static void Main(string[] args){    string txt = "BACDGABCDA";    string pat = "ABCD";    search(pat, txt);}} // This code is contributed// by Shrikant1



Output:

Found at Index 0
Found at Index 5
Found at Index 6