Queries on subsequence of string

Last Updated : 12 Sep, 2023

Given a string S and Q queries, each query contains a string T. The task is to print "Yes" if T is a subsequence of S, else print "No".

Examples:

Input : S = "geeksforgeeks"
Query 1: "gg"
Query 2: "gro"
Query 3: "gfg"
Query 4: "orf"

Output :
Yes
No
Yes
No

For each query, using the brute force, start iterating over S looking for the first character of T. As soon as, the first character is found, continue to iterate S now looking for the second character of T and so on (Refer this for details). If manage to find all the character of T, print "Yes", else "No". Time complexity is O(Q*N), N is the length of S.

The efficient approach can be if we know the position of the next character of T in S. Then simply skip all the character between current and position of the next character and jump to that position. This can be done by making |S| x 26 size matrix and storing the next position of each character from every position of S.

Below is the implementation of the above idea :

C++

// C++ program to answer subsequence queries for a
// given string.
#include <bits/stdc++.h>
#define MAX 10000
#define CHAR_SIZE 26
using namespace std;

// Precompute the position of each character from
// each position of String S
void precompute(int mat[MAX][CHAR_SIZE], char str[],
                                           int len)
{
    for (int i = 0; i < CHAR_SIZE; ++i)
        mat[len][i] = len;

    // Computing position of each character from
    // each position of String S
    for (int i = len-1; i >= 0; --i)
    {
        for (int j = 0; j < CHAR_SIZE; ++j)
            mat[i][j] = mat[i+1][j];

        mat[i][str[i]-'a'] = i;
    }
}

// Print "Yes" if T is subsequence of S, else "No"
bool query(int mat[MAX][CHAR_SIZE], const char *str, 
                                          int len)
{
    int pos = 0;

    // Traversing the string T
    for (int i = 0; i < strlen(str); ++i)
    {
        // If next position is greater than 
        // length of S set flag to false.
        if (mat[pos][str[i] - 'a'] >= len)
            return false;

        // Setting position of next character
        else
            pos = mat[pos][str[i] - 'a'] + 1;
    }
    return true;
}

// Driven Program
int main()
{
    char S[]= "geeksforgeeks";
    int len = strlen(S);

    int mat[MAX][CHAR_SIZE];
    precompute(mat, S, len);

    query(mat, "gg", len)?  cout << "Yes\n" :
                            cout << "No\n";
    query(mat, "gro", len)? cout << "Yes\n" :
                            cout << "No\n";
    query(mat, "gfg", len)? cout << "Yes\n" :
                            cout << "No\n";
    query(mat, "orf", len)? cout << "Yes\n" :
                            cout << "No\n";

    return 0;
}

Java

// Java program to answer subsequence queries for
// a given string.
public class Query_Subsequence {

    static final int MAX = 10000;
    static final int CHAR_SIZE = 26;
     
    // Precompute the position of each character from
    // each position of String S
    static void precompute(int mat[][], String str, int len)
    {
        for (int i = 0; i < CHAR_SIZE; ++i)
            mat[len][i] = len;
     
        // Computing position of each character from
        // each position of String S
        for (int i = len-1; i >= 0; --i)
        {
            for (int j = 0; j < CHAR_SIZE; ++j)
                mat[i][j] = mat[i+1][j];
     
            mat[i][str.charAt(i)-'a'] = i;
        }
    }
     
    // Print "Yes" if T is subsequence of S, else "No"
    static boolean query(int mat[][], String str, int len)
    {
        int pos = 0;
     
        // Traversing the string T
        for (int i = 0; i < str.length(); ++i)
        {
            // If next position is greater than 
            // length of S set flag to false.
            if (mat[pos][str.charAt(i) - 'a'] >= len)
                return false;
     
            // Setting position of next character
            else
                pos = mat[pos][str.charAt(i) - 'a'] + 1;
        }
        return true;
    }
     
    // Driven Program
    public static void main(String args[])
    {
        String S= "geeksforgeeks";
        int len = S.length();
     
        int[][] mat = new int[MAX][CHAR_SIZE];
        precompute(mat, S, len);
             
        String get = query(mat, "gg", len)? "Yes" :"No";
        System.out.println(get);
        get = query(mat, "gro", len)? "Yes" :"No";
        System.out.println(get);
        get = query(mat, "gfg", len)? "Yes" :"No";
        System.out.println(get);
        get = query(mat, "orf", len)? "Yes" :"No";
        System.out.println(get);
     
    }
}
// This code is contributed by Sumit Ghosh

Python3

# Python3 program to answer 
# subsequence queries for 
# a given string.
MAX = 10000
CHAR_SIZE = 26

# Precompute the position of 
# each character from 
# each position of String S 
def precompute(mat, str, Len):

    for i in range(CHAR_SIZE):
        mat[Len][i] = Len

    # Computing position of each 
    # character from each position 
    # of String S 
    for i in range(Len - 1, -1, -1):
        for j in range(CHAR_SIZE):
            mat[i][j] = mat[i + 1][j]
        mat[i][ord(str[i]) - 
               ord('a')] = i

# Print "Yes" if T is
# subsequence of S, else "No" 
def query(mat, str, Len):
    pos = 0

    # Traversing the string T 
    for i in range(len(str)):

        # If next position is greater than  
        # length of S set flag to false. 
        if(mat[pos][ord(str[i]) - 
                    ord('a')] >= Len):
            return False

        # Setting position of next character 
        else:
            pos = mat[pos][ord(str[i]) - 
                           ord('a')] + 1
    return True

# Driven code
S = "geeksforgeeks"
Len = len(S)
mat = [[0 for i in range(CHAR_SIZE)] 
          for j in range(MAX)]
precompute(mat, S, Len)

get = "No"
if(query(mat, "gg", Len)):
    get = "Yes"
print(get)

get = "No"
if(query(mat, "gro", Len)):
    get = "Yes"
print(get)

get = "No"
if(query(mat, "gfg", Len)):
    get = "Yes"
print(get)

get = "No"
if(query(mat, "orf", Len)):
    get = "Yes"
print(get)

# This code is contributed by avanitrachhadiya2155

// C# program to answer subsequence
// queries for a given string
using System;
public class Query_Subsequence 
{

    static int MAX = 10000;
    static int CHAR_SIZE = 26;
    
    // Precompute the position of each 
    // character from each position
    // of String S
    static void precompute(int [,]mat, 
                        string str, 
                        int len)
    {
        
        for (int i = 0; i < CHAR_SIZE; ++i)
            mat[len, i] = len;
    
        // Computing position of each 
        // character from each position
        // of String S
        for (int i = len - 1; i >= 0; --i)
        {
            for (int j = 0; j < CHAR_SIZE;
                ++j)
                mat[i, j] = mat[i + 1, j];
    
            mat[i, str[i] - 'a'] = i;
        }
    }
    
    // Print "Yes" if T is subsequence
    // of S, else "No"
    static bool query(int [,]mat, 
                    string str, 
                    int len)
    {
        int pos = 0;
    
        // Traversing the string T
        for (int i = 0; i < str.Length; ++i)
        {
            // If next position is greater than 
            // length of S set flag to false.
            if (mat[pos,str[i] - 'a'] >= len)
                return false;
    
            // Setting position of next character
            else
                pos = mat[pos,str[i] - 'a'] + 1;
        }
        return true;
    }
    
    // Driver Code
    public static void Main()
    {
        string S= "geeksforgeeks";
        int len = S.Length;
    
        int[,] mat = new int[MAX,CHAR_SIZE];
        precompute(mat, S, len);
            
        string get = query(mat, "gg", len)? 
                           "Yes" :"No";
        Console.WriteLine(get);
        get = query(mat, "gro", len)? 
                         "Yes" :"No";
        Console.WriteLine(get);
        get = query(mat, "gfg", len)? 
                         "Yes" :"No";
        Console.WriteLine(get);
        get = query(mat, "orf", len)? 
                         "Yes" :"No";
        Console.WriteLine(get);
    
    }
}

// This code is contributed by vt_m.

JavaScript

<script>
    // Javascript program to answer subsequence queries for
    // a given string.
    
    let MAX = 10000;
    let CHAR_SIZE = 26;
      
    // Precompute the position of each character from
    // each position of String S
    function precompute(mat, str, len)
    {
        for (let i = 0; i < CHAR_SIZE; ++i)
            mat[len][i] = len;
      
        // Computing position of each character from
        // each position of String S
        for (let i = len-1; i >= 0; --i)
        {
            for (let j = 0; j < CHAR_SIZE; ++j)
                mat[i][j] = mat[i+1][j];
      
            mat[i][str[i].charCodeAt()-'a'.charCodeAt()] = i;
        }
    }
      
    // Print "Yes" if T is subsequence of S, else "No"
    function query(mat, str, len)
    {
        let pos = 0;
      
        // Traversing the string T
        for (let i = 0; i < str.length; ++i)
        {
            // If next position is greater than
            // length of S set flag to false.
            if (mat[pos][str[i].charCodeAt() - 'a'.charCodeAt()] >= len)
                return false;
      
            // Setting position of next character
            else
                pos = mat[pos][str[i].charCodeAt() - 'a'.charCodeAt()] + 1;
        }
        return true;
    }
    
    let S= "geeksforgeeks";
    let len = S.length;

    let mat = new Array(MAX);
    for(let i = 0; i < MAX; i++)
    {
        mat[i] = new Array(CHAR_SIZE);
        for(let j = 0; j < CHAR_SIZE; j++)
        {
            mat[i][j] = 0;
        }
    }
    precompute(mat, S, len);

    let get = query(mat, "gg", len)? "Yes" :"No";
    document.write(get + "</br>");
    get = query(mat, "gro", len)? "Yes" :"No";
    document.write(get + "</br>");
    get = query(mat, "gfg", len)? "Yes" :"No";
    document.write(get + "</br>");
    get = query(mat, "orf", len)? "Yes" :"No";
    document.write(get + "</br>");
    
</script>

Output

Yes
No
Yes
No

Queries on subsequence of string

Anuj Chauhan

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Queries on subsequence of string

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