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Longest sub-sequence of a binary string divisible by 3
  • Difficulty Level : Easy
  • Last Updated : 14 Jan, 2020

Given a binary string S of length N, the task is to find the length of the longest sub-sequence in it which is divisible by 3. Leading zeros in the sub-sequences are allowed.

Examples:

Input: S = “1001”
Output: 4
The longest sub-sequence divisible by 3 is “1001”.
1001 = 9 which is divisible by 3.

Input: S = “1011”
Output: 3

Naive approach: Generate all the possible sub-sequences and check if they are divisible by 3. The time complexity for this will be O((2N) * N).



Efficient approach: Dynamic programming can be used to solve this problem. Let’s look at the states of DP.
DP[i][r] will store the longest sub-sequence of the substring S[i…N-1] such that it gives a remainder of (3 – r) % 3 when divided by 3.
Let’s write the recurrence relation now.

DP[i][r] = max(1 + DP[i + 1][(r * 2 + s[i]) % 3], DP[i + 1][r])

The recurrence is derived because of the following two choices:

  1. Include the current index i in the sub-sequence. Thus, the r will be updated as r = (r * 2 + s[i]) % 3.
  2. Don’t include the current index in the sub-sequence.

Below is the implementation of the above approach:

C++




// C++ implementation of the approach
#include <bits/stdc++.h>
using namespace std;
#define N 100
  
int dp[N][3];
bool v[N][3];
  
// Function to return the length of the
// largest sub-string divisible by 3
int findLargestString(string& s, int i, int r)
{
    // Base-case
    if (i == s.size()) {
        if (r == 0)
            return 0;
        else
            return INT_MIN;
    }
  
    // If the state has been solved
    // before then return its value
    if (v[i][r])
        return dp[i][r];
  
    // Marking the state as solved
    v[i][r] = 1;
  
    // Recurrence relation
    dp[i][r]
        = max(1 + findLargestString(s, i + 1,
                                    (r * 2 + (s[i] - '0')) % 3),
              findLargestString(s, i + 1, r));
    return dp[i][r];
}
  
// Driver code
int main()
{
    string s = "101";
  
    cout << findLargestString(s, 0, 0);
  
    return 0;
}

Java




// Java implementation of th approach 
class GFG 
{
  
    final static int N = 100 ;
    final static int INT_MIN = Integer.MIN_VALUE;
      
    static int dp[][] = new int[N][3]; 
    static int v[][] = new int[N][3]; 
      
      
    // Function to return the length of the 
    // largest sub-string divisible by 3 
    static int findLargestString(String s, int i, int r) 
    
        // Base-case 
        if (i == s.length())
        
            if (r == 0
                return 0
            else
                return INT_MIN; 
        
      
        // If the state has been solved 
        // before then return its value 
        if (v[i][r] == 1
            return dp[i][r]; 
      
        // Marking the state as solved 
        v[i][r] = 1
      
        // Recurrence relation 
        dp[i][r] = Math.max(1 + findLargestString(s, i + 1
                          (r * 2 + (s.charAt(i) - '0')) % 3), 
                            findLargestString(s, i + 1, r)); 
        return dp[i][r];
    }
      
    // Driver code 
    public static void main (String[] args) 
    
        String s = "101"
      
        System.out.print(findLargestString(s, 0, 0)); 
    
}
  
// This code is contributed by AnkitRai01

Python3




# Python3 implementation of the approach 
import numpy as np
import sys
  
N = 100
INT_MIN = -(sys.maxsize - 1)
  
dp = np.zeros((N, 3)); 
v = np.zeros((N, 3)); 
  
# Function to return the length of the 
# largest sub-string divisible by 3 
def findLargestString(s, i, r) : 
  
    # Base-case 
    if (i == len(s)) :
        if (r == 0) :
            return 0
        else :
            return INT_MIN; 
  
    # If the state has been solved 
    # before then return its value 
    if (v[i][r]) :
        return dp[i][r]; 
  
    # Marking the state as solved 
    v[i][r] = 1
  
    # Recurrence relation 
    dp[i][r] = max(1 + findLargestString(s, i + 1
                  (r * 2 + (ord(s[i]) - ord('0'))) % 3),
                       findLargestString(s, i + 1, r)); 
                  
    return dp[i][r]; 
  
# Driver code 
if __name__ == "__main__"
  
    s = "101"
  
    print(findLargestString(s, 0, 0)); 
  
# This code is contributed by AnkitRai01

C#




// C# implementation of th approach 
using System;
using System.Collections.Generic;
  
class GFG 
{
  
    readonly static int N = 100 ;
    readonly static int INT_MIN = int.MinValue;
      
    static int [,]dp = new int[N, 3]; 
    static int [,]v = new int[N, 3]; 
      
    // Function to return the length of the 
    // largest sub-string divisible by 3 
    static int findLargestString(String s, int i, int r) 
    
        // Base-case 
        if (i == s.Length)
        
            if (r == 0) 
                return 0; 
            else
                return INT_MIN; 
        
      
        // If the state has been solved 
        // before then return its value 
        if (v[i, r] == 1) 
            return dp[i, r]; 
      
        // Marking the state as solved 
        v[i, r] = 1; 
      
        // Recurrence relation 
        dp[i, r] = Math.Max(1 + findLargestString(s, i + 1, 
                                (r * 2 + (s[i] - '0')) % 3), 
                            findLargestString(s, i + 1, r)); 
        return dp[i, r];
    }
      
    // Driver code 
    public static void Main(String[] args) 
    
        String s = "101"
      
        Console.Write(findLargestString(s, 0, 0)); 
    
}
  
// This code is contributed by 29AjayKumar
Output:
2

Time Complexity: O(n)

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