Queries on subsequence of string

2.7

Given a string S and Q queries, each query contains a string T. The task is print “Yes” if T is 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 be O(Q*N), N is the length of S.

The efficient approach can be if we know the position of next character of T in S. Then simply skip all the character between current and position of 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 this approach:

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


Output:

Yes
No
Yes
No

This article is contributed by Anuj Chauhan. 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.

Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.

GATE CS Corner    Company Wise Coding Practice

Please write to us at contribute@geeksforgeeks.org to report any issue with the above content.

Recommended Posts:



2.7 Average Difficulty : 2.7/5.0
Based on 7 vote(s)










Writing code in comment? Please use ide.geeksforgeeks.org, generate link and share the link here.