Length of largest subsequence consisting of a pair of alternating digits

Given a numeric string s consisting of digits 0 to 9, the task is to find the length of the largest subsequence consisting of a pair of alternating digits.

An alternating digits subsequence consisting of two different digits a and b can be represented as “abababababababababab….”.

Examples:

Input: s = “1542745249842”
Output: 6
Explanation: 
The largest substring of alternating digits in the given string is 424242.

Input:  s = “1212312323232”
Output: 9
Explanation:
The largest substring of alternating digits in the given string is 232323232.



Approach: The string consists of only decimal digits i.e., 0-9, thus the sequence can be checked for the presence of all possible subsequences consisting of two alternating digits. For this purpose follow the below approach:

Below is the implementation of the above approach:

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// C++ program for the above approach
#include <iostream>
using namespace std;
 
// Function to find the length of the
// largest subsequence consisting of
// a pair of alternating digits
void largestSubsequence(string s)
{
    // Variable initialization
    int maxi = 0;
    char prev1;
 
    // Nested loops for iteration
    for (int i = 0; i < 10; i++) {
        for (int j = 0; j < 10; j++) {
 
            // Check if i is not eqaul to j
            if (i != j) {
 
                // Initialize length as 0
                int len = 0;
                prev1 = j + '0';
 
                // Iterate from 0 till the
                // size of the string
                for (int k = 0; k < s.size(); k++) {
 
                    if (s[k] == i + '0'
                        && prev1 == j + '0') {
                        prev1 = s[k];
 
                        // Increment length
                        len++;
                    }
                    else if (s[k] == j + '0'
                             && prev1 == i + '0') {
                        prev1 = s[k];
 
                        // Increment length
                        len++;
                    }
                }
 
                // Update maxi
                maxi = max(len, maxi);
            }
        }
    }
 
    // Check if maxi is not equal to
    // 1 the print it otherwise print 0
    if (maxi != 1)
        cout << maxi << endl;
    else
        cout << 0 << endl;
}
 
// Driver Code
int main()
{
    // Given string
    string s = "1542745249842";
 
    // Function call
    largestSubsequence(s);
    return 0;
}
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// Java program for the above approach
import java.util.*;
class GFG{
 
// Function to find the length of the
// largest subsequence consisting of
// a pair of alternating digits
static void largestSubsequence(char []s)
{
    // Variable initialization
    int maxi = 0;
    char prev1;
 
    // Nested loops for iteration
    for (int i = 0; i < 10; i++)
    {
        for (int j = 0; j < 10; j++)
        {
 
            // Check if i is not eqaul to j
            if (i != j)
            {
 
                // Initialize length as 0
                int len = 0;
                prev1 = (char) (j + '0');
 
                // Iterate from 0 till the
                // size of the String
                for (int k = 0; k < s.length; k++)
                {
                    if (s[k] == i + '0' &&
                        prev1 == j + '0')
                    {
                        prev1 = s[k];
 
                        // Increment length
                        len++;
                    }
                    else if (s[k] == j + '0' &&
                             prev1 == i + '0')
                    {
                        prev1 = s[k];
 
                        // Increment length
                        len++;
                    }
                }
 
                // Update maxi
                maxi = Math.max(len, maxi);
            }
        }
    }
 
    // Check if maxi is not equal to
    // 1 the print it otherwise print 0
    if (maxi != 1)
        System.out.print(maxi + "\n");
    else
        System.out.print(0 + "\n");
}
 
// Driver Code
public static void main(String[] args)
{
    // Given String
    String s = "1542745249842";
 
    // Function call
    largestSubsequence(s.toCharArray());
}
}
 
// This code is contributed by Rohit_ranjan
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# Python3 program for the above approach
 
# Function to find the length of the
# largest subsequence consisting of
# a pair of alternating digits
def largestSubsequence(s):
     
    # Variable initialization
    maxi = 0
 
    # Nested loops for iteration
    for i in range(10):
        for j in range(10):
 
            # Check if i is not eqaul to j
            if (i != j):
 
                # Initialize length as 0
                lenn = 0
                prev1 = chr(j + ord('0'))
 
                # Iterate from 0 till the
                # size of the string
                for k in range(len(s)):
                    if (s[k] == chr(i + ord('0')) and
                       prev1 == chr(j + ord('0'))):
                        prev1 = s[k]
 
                        # Increment length
                        lenn += 1
                         
                    elif (s[k] == chr(j + ord('0')) and
                         prev1 == chr(i + ord('0'))):
                        prev1 = s[k]
 
                        # Increment lenngth
                        lenn += 1
 
                # Update maxi
                maxi = max(lenn, maxi)
 
    # Check if maxi is not equal to
    # 1 the prit otherwise pr0
    if (maxi != 1):
        print(maxi)
    else:
        print(0)
 
# Driver Code
if __name__ == '__main__':
     
    # Given string
    s = "1542745249842"
 
    # Function call
    largestSubsequence(s)
 
# This code is contributed by mohit kumar 29
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// C# program for the above approach
using System;
class GFG{
 
// Function to find the length of the
// largest subsequence consisting of
// a pair of alternating digits
static void largestSubsequence(char []s)
{
    // Variable initialization
    int maxi = 0;
    char prev1;
 
    // Nested loops for iteration
    for (int i = 0; i < 10; i++)
    {
        for (int j = 0; j < 10; j++)
        {
 
            // Check if i is not eqaul to j
            if (i != j)
            {
 
                // Initialize length as 0
                int len = 0;
                prev1 = (char) (j + '0');
 
                // Iterate from 0 till the
                // size of the String
                for (int k = 0; k < s.Length; k++)
                {
                    if (s[k] == i + '0' &&
                        prev1 == j + '0')
                    {
                        prev1 = s[k];
 
                        // Increment length
                        len++;
                    }
                    else if (s[k] == j + '0' &&
                             prev1 == i + '0')
                    {
                        prev1 = s[k];
 
                        // Increment length
                        len++;
                    }
                }
 
                // Update maxi
                maxi = Math.Max(len, maxi);
            }
        }
    }
 
    // Check if maxi is not equal to
    // 1 the print it otherwise print 0
    if (maxi != 1)
        Console.Write(maxi + "\n");
    else
        Console.Write(0 + "\n");
}
 
// Driver Code
public static void Main(String[] args)
{
    // Given String
    String s = "1542745249842";
 
    // Function call
    largestSubsequence(s.ToCharArray());
}
}
 
// This code is contributed by Rohit_ranjan
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Output: 
6





 

Time Complexity: O(10*10*N) 
Auxiliary Space: O(1)

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Improved By : mohit kumar 29, Rohit_ranjan

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