Check if a string is a subsequence of another string ( using Stacks )

Given a string S, the task is to check if the string target is a subsequence of string S or not, using a Stack.

Examples:

Input: S = ”KOTTAYAM”, target = ”KOTA”
Output: Yes
Explanation: “KOTA” is a subsequence of “KOTTAYAM”.

Input: S = ”GEEKSFORGEEKS”, target =”FORFOR”
Output: No

Approach: Follow the steps to solve the problem:

• Initialize a stack s.
• Iterate over the characters of the string target.

target pushed into the stack

• Traverse the string S in the reverse.
• If the current character of string S is same as the character at the top of the stack, pop the top element of the stack.

Traversing in S

Traversing in S

Popping from stack

Traversing in S

Popping from stack

Traversing in st

Popping from stack

Traversing in S

Stack becomes empty

• If at any point, the stack becomes empty, it can be concluded that target is a subsequence of S.
• Otherwise, target is not a subsequence of S.

Below is the implementation of the above approach:

C++

// C++ Program for the above approach

#include <bits/stdc++.h>
using namespace std;

// Function to check if target
// is a subsequence of string S
void checkforSubsequence(string S,
                         string target)
{

    // Declare a stack
    stack<char> s;

    // Push the characters of
    // target into the stack
    for (int i = 0; i < target.size(); i++) {
        s.push(target[i]);
    }

    // Traverse the string S in reverse
    for (int i = (int)S.size() - 1; i >= 0; i--) {

        // If the stack is empty
        if (s.empty()) {

            cout << "Yes" << endl;
            return;
        }

        // if S[i] is same as the
        // top of the stack
        if (S[i] == s.top()) {

            // Pop the top of stack
            s.pop();
        }
    }

    // Stack s is empty
    if (s.empty())
        cout << "Yes" << endl;
    else
        cout << "No" << endl;
}

// Driver Code
int main()
{
    string S = "KOTTAYAM";
    string target = "KOTA";

    checkforSubsequence(S, target);

    return 0;
}

Java

// Java approach for the above approach
import java.util.Stack;

public class GFG {

    // Function to check if target
    // is a subsequence of string S
    static void checkforSubsequence(String S, String target)
    {

        // Declare a stack
        Stack<Character> s = new Stack<>();

        // Push the characters of
        // target into the stack
        for (int i = 0; i < target.length(); i++) {
            s.push(target.charAt(i));
        }

        // Traverse the string S in reverse
        for (int i = (int)S.length() - 1; i >= 0; i--) {

            // If the stack is empty
            if (s.empty()) {

                System.out.println("Yes");
                return;
            }

            // if S[i] is same as the
            // top of the stack
            if (S.charAt(i) == s.peek()) {

                // Pop the top of stack
                s.pop();
            }
        }

        // Stack s is empty
        if (s.empty())
            System.out.println("Yes");
        else
            System.out.println("No");
    }

    // Driver Code
    public static void main(String[] args)
    {
        String S = "KOTTAYAM";
        String target = "KOTA";

        checkforSubsequence(S, target);
    }
}

// This code is contributed by abhinavjain194

Python3

# Python3 program for the above approach

# Function to check if target
# is a subsequence of string S
def checkforSubsequence(S, target):

    # Declare a stack
    s = []

    # Push the characters of
    # target into the stack
    for i in range(len(target)):
        s.append(target[i])

    # Traverse the string S in reverse
    for i in range(len(S) - 1, -1, -1):

        # If the stack is empty
        if (len(s) == 0):
            print("Yes")
            return

        # If S[i] is same as the
        # top of the stack
        if (S[i] == s[-1]):

            # Pop the top of stack
            s.pop()

    # Stack s is empty
    if (len(s) == 0):
        print("Yes")
    else:
        print("No")

# Driver Code
if __name__ == "__main__":

    S = "KOTTAYAM"
    target = "KOTA"

    checkforSubsequence(S, target)

# This code is contributed by ukasp

C#

// C# approach for the above approach
using System;
using System.Collections.Generic;

class GFG{

// Function to check if target
// is a subsequence of string S
static void checkforSubsequence(String S,
                                String target)
{

    // Declare a stack
    Stack<char> s = new Stack<char>();

    // Push the characters of
    // target into the stack
    for(int i = 0; i < target.Length; i++)
    {
        s.Push(target[i]);
    }

    // Traverse the string S in reverse
    for(int i = (int)S.Length - 1; i >= 0; i--)
    {

        // If the stack is empty
        if (s.Count == 0)
        {
            Console.WriteLine("Yes");
            return;
        }

        // If S[i] is same as the
        // top of the stack
        if (S[i] == s.Peek())
        {

            // Pop the top of stack
            s.Pop();
        }
    }

    // Stack s is empty
    if (s.Count == 0)
        Console.WriteLine("Yes");
    else
        Console.WriteLine("No");
}

// Driver Code
public static void Main(String[] args)
{
    String S = "KOTTAYAM";
    String target = "KOTA";

    checkforSubsequence(S, target);
}
}

// This code is contributed by shikhasingrajput

Javascript

<script>

// JavaScript Program for the above approach

// Function to check if target
// is a subsequence of string S
function checkforSubsequence(S, target)
{

    // Declare a stack
    var s = [];

    // Push the characters of
    // target into the stack
    for (var i = 0; i < target.length; i++) {
        s.push(target[i]);
    }

    // Traverse the string S in reverse
    for (var i = S.length - 1; i >= 0; i--) {

        // If the stack is empty
        if (s.length==0) {

            document.write( "Yes");
            return;
        }

        // if S[i] is same as the
        // top of the stack
        if (S[i] == s[s.length-1]) {

            // Pop the top of stack
            s.pop();
        }
    }

    // Stack s is empty
    if (s.length==0)
        document.write( "Yes" );
    else
        document.write( "No" );
}

// Driver Code

var S = "KOTTAYAM";
var target = "KOTA";
checkforSubsequence(S, target);


</script>

Output

Yes

Time Complexity : O(N) 
Auxiliary Space : O(N)

Approach 2: Using the find() function

Another approach to check if a string is a subsequence of another string is to use the find() function. We can call find() on the second string to find the index of the first occurrence of the first character of the first string. Then, we can iterate over the first string and call find() on the second string with the starting index set to the index of the previous character plus one. If find() returns string::npos, it means that the current character in the first string is not present in the second string and hence the first string is not a subsequence of the second string.

    Define a function that takes two string arguments s1 and s2.

   Initialize an integer variable index to -1. This will be used to keep track of the starting index for each find() call.

   Iterate over each character c in s1 using a range-based for loop.

   Call the find() function on s2 with the arguments c and index + 1. The second argument specifies the starting index for the search, which is the index of the previous character plus one.

   If find() returns string::npos, which indicates that the character was not found in s2, return false immediately, as s1 is not a subsequence of s2.

   If find() returns a valid index, update index to the returned value.

   After iterating over all the characters in s1, return true, as s1 is a subsequence of s2.

C++

#include <iostream>
#include <string>

using namespace std;

bool isSubsequence(string s1, string s2) {
    int index = -1;
    for (char c : s1) {
        index = s2.find(c, index + 1);
        if (index == string::npos) {
            return false;
        }
    }
    return true;
}

int main() {
    string s1 = "KOTA";
    string s2 = "KOTTAYAM";
    bool check =  isSubsequence(s1, s2) ;
    if(check){
        cout<<" yes";
    }
    else{
        cout<<"no";
    }
    return 0;
}

Java

import java.util.*;

public class GFG {

    public static boolean isSubsequence(String s1, String s2) {
      // Initialize index to -1
      int index = -1;

      // Loop through each character in s1
        for (char c : s1.toCharArray()) {

          // Find the index of the current character in s2
            index = s2.indexOf(c, index + 1);
          // If the character is not found, return false
            if (index == -1) {
                return false;
            }
        }
        return true; // All characters are found, return true
    }

 // Driver Code
    public static void main(String[] args) {
     // input taken
        String s1 = "KOTA";
        String s2 = "KOTTAYAM";
        boolean check = isSubsequence(s1, s2);
      // Print "yes" if s1 is a subsequence of s2
        if (check) {
            System.out.println("yes");
        }
      // Print "no" if s1 is not a subsequence of s2
      else {
            System.out.println("no");
        }
    }
}

Python3

def isSubsequence(s1, s2):
    index = -1
    for c in s1:
        index = s2.find(c, index + 1)
        if index == -1:
            return False
    return True

s1 = "KOTA"
s2 = "KOTTAYAM"
check = isSubsequence(s1, s2)
if check:
    print("yes")
else:
    print("no")

C#

using System;

public class Program
{
    // Function to check if s1 is a subsequence of s2
    static bool IsSubsequence(string s1, string s2)
    {
        int index = -1;

        foreach (char c in s1)
        {
            // Search for the character 'c' in s2, starting from index + 1
            index = s2.IndexOf(c, index + 1);

            // If 'c' is not found in s2, return false
            if (index == -1)
            {
                return false;
            }
        }

        return true;
    }

    public static void Main()
    {
        string s1 = "KOTA";
        string s2 = "KOTTAYAM";

        // Check if s1 is a subsequence of s2
        bool check = IsSubsequence(s1, s2);

        // Print the result
        if (check)
        {
            Console.WriteLine("Yes, s1 is a subsequence of s2");
        }
        else
        {
            Console.WriteLine("No, s1 is not a subsequence of s2");
        }
    }
}

Javascript

function isSubsequence(s1, s2) {
    // Initialize index to -1
    let index = -1;

    // Loop through each character in s1
    for (const c of s1) {
        // Find the index of the current character in s2
        index = s2.indexOf(c, index + 1);

        // If the character is not found, return false
        if (index === -1) {
            return false;
        }
    }
    return true; // All characters are found, return true
}

// Driver Code
const s1 = "KOTA";
const s2 = "KOTTAYAM";
const check = isSubsequence(s1, s2);

// Print "yes" if s1 is a subsequence of s2
if (check) {
    console.log("yes");
}
// Print "no" if s1 is not a subsequence of s2
else {
    console.log("no");
}

// This code is contributed by shivamgupta310570

Output

 yes

Time Complexity : O(M*N) 
Auxiliary Space : O(1)