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Maximum number of strings that can be formed with given zeros and ones
  • Difficulty Level : Hard
  • Last Updated : 09 Apr, 2020

Given a list of strings arr[] of zeros and ones only and two integer N and M, where N is the number of 1’s and M is the number of 0’s. The task is to find the maximum number of strings from the given list of strings that can be constructured with given number of 0’s and 1’s.

Examples:

Input: arr[] = {“10”, “0001”, “11100”, “1”, “0”}, M = 5, N = 3
Output: 4
Explanation:
The 4 strings which can be formed using five 0’s and three 1’s are: “10”, “0001”, “1”, “0”

Input: arr[] = {“10”, “00”, “000” “0001”, “111001”, “1”, “0”}, M = 3, N = 1
Output: 3
Explanation:
The 3 strings which can be formed using three 0’s and one 1’s are: “00”, “1”, “0”

Naive Approach: The idea is to generate all the combination of the given list of strings and check the count of zeros and ones satisfying the given condition. But the time complexity of this solution is exponential.



Time Complexity: O(2N), where N is the number of strings in the list.

Efficient Approach:
An efficient solution is given by using Dynamic Programming. The idea is to use recursion for generating all possible combinations and store the results for Overlapping Subproblems during recursion.
Below are the steps:

  1. The idea is to use 3D dp array(dp[M][N][i]) where N and M are the number of 1’s and 0’s respectively and i is the index of the string in the list.
  2. Find the number of 1’s and 0’s in the current string and check if the count of the zeros and ones is less than or equals to the given count N and M respectively.
  3. If above condition is true, then check whether current state value is stored in the dp table or not. If yes then return this value.
  4. Else recursively move for the next iteration by including and excluding the current string as:
    // By including the current string
    x = 1 + recursive_function(M - zero, N - ones, arr, i + 1)
    
    // By excluding the current string 
    y = recursive_function(M, N, arr, i + 1)
    
    // and update the dp table as:
    dp[M][N][i] = max(x, y)
    
  5. The maximum value of the above two recursive calls will give the maximum number with N 1’s and M 0’s for the current state.

Below is the implementation of the above approach:

C++




// C++ program for the above approach
#include <bits/stdc++.h>
using namespace std;
  
// 3D dp table to store the state value
int dp[100][100][100];
  
// Function that count the combination
// of 0's and 1's from the given list
// of string
int countString(int m, int n,
                vector<string>& arr, int i)
{
    // Base Case if count of 0's or 1's
    // becomes negative
    if (m < 0 || n < 0) {
        return INT_MIN;
    }
  
    // If index reaches out of bound
    if (i >= arr.size()) {
        return 0;
    }
  
    // Return the prestored result
    if (dp[m][n][i] != -1) {
        return dp[m][n][i];
    }
  
    // Intialise count of 0's and 1's
    // to 0 for the current state
    int zero = 0, one = 0;
  
    // Calculate the number of 1's and
    // 0's in current string
    for (char c : arr[i]) {
        if (c == '0') {
            zero++;
        }
        else {
            one++;
        }
    }
  
    // Include the current string and
    // recurr for the next iteration
    int x = 1 + countString(m - zero,
                            n - one,
                            arr, i + 1);
  
    // Exclude the current string and
    // recurr for the next iteration
    int y = countString(m, n, arr, i + 1);
  
    // Update the maximum of the above
    // two states to the current dp state
    return dp[m][n][i] = max(x, y);
}
  
// Driver Code
int main()
{
    vector<string> arr = { "10", "0001", "1",
                           "111001", "0" };
  
    // N 0's and M 1's
    int N = 3, M = 5;
  
    // Intialise dp array to -1
    memset(dp, -1, sizeof(dp));
  
    // Function call
    cout << countString(M, N, arr, 0);
}

Java




// Java program for the above approach
class GFG{
   
// 3D dp table to store the state value
static int [][][]dp = new int[100][100][100];
   
// Function that count the combination
// of 0's and 1's from the given list
// of String
static int countString(int m, int n,
                String []arr, int i)
{
    // Base Case if count of 0's or 1's
    // becomes negative
    if (m < 0 || n < 0) {
        return Integer.MIN_VALUE;
    }
   
    // If index reaches out of bound
    if (i >= arr.length) {
        return 0;
    }
   
    // Return the prestored result
    if (dp[m][n][i] != -1) {
        return dp[m][n][i];
    }
   
    // Intialise count of 0's and 1's
    // to 0 for the current state
    int zero = 0, one = 0;
   
    // Calculate the number of 1's and
    // 0's in current String
    for (char c : arr[i].toCharArray()) {
        if (c == '0') {
            zero++;
        }
        else {
            one++;
        }
    }
   
    // Include the current String and
    // recurr for the next iteration
    int x = 1 + countString(m - zero,
                            n - one,
                            arr, i + 1);
   
    // Exclude the current String and
    // recurr for the next iteration
    int y = countString(m, n, arr, i + 1);
   
    // Update the maximum of the above
    // two states to the current dp state
    return dp[m][n][i] = Math.max(x, y);
}
   
// Driver Code
public static void main(String[] args)
{
    String []arr = { "10", "0001", "1",
                           "111001", "0" };
   
    // N 0's and M 1's
    int N = 3, M = 5;
   
    // Intialise dp array to -1
    for(int i = 0;i<100;i++){
        for(int j = 0;j<100;j++){
            for(int l=0;l<100;l++)
            dp[i][j][l]=-1;
        }
    }
   
    // Function call
    System.out.print(countString(M, N, arr, 0));
}
}
  
// This code is contributed by 29AjayKumar

Python 3




# Python 3 program for the above approach
import sys
  
# 3D dp table to store the state value
dp = [[[-1 for i in range(100)]for j in range(100)] for k in range(100)]
  
# Function that count the combination
# of 0's and 1's from the given list
# of string
def countString(m, n, arr, i):
      
    # Base Case if count of 0's or 1's
    # becomes negative
    if (m < 0 or n < 0):
        return -sys.maxsize - 1
  
    # If index reaches out of bound
    if (i >= len(arr)):
        return 0
  
    # Return the prestored result
    if (dp[m][n][i] != -1):
        return dp[m][n][i]
  
    # Intialise count of 0's and 1's
    # to 0 for the current state
    zero = 0
    one = 0
  
    # Calculate the number of 1's and
    # 0's in current string
    for c in arr[i]:
        if (c == '0'):
            zero += 1
        else:
            one += 1
  
    # Include the current string and
    # recurr for the next iteration
    x = 1 + countString(m - zero, n - one, arr, i + 1)
  
    # Exclude the current string and
    # recurr for the next iteration
    y = countString(m, n, arr, i + 1)
      
    dp[m][n][i] = max(x, y)
  
    # Update the maximum of the above
    # two states to the current dp state
    return dp[m][n][i]
  
# Driver Code
if __name__ == '__main__':
    arr = ["10", "0001", "1","111001", "0"]
  
    # N 0's and M 1's
    N = 3
    M = 5
  
    # Function call
    print(countString(M, N, arr, 0))
      
# This code is contributed by Surendra_Gangwar

C#




// C# program for the above approach
using System;
  
class GFG{
    
// 3D dp table to store the state value
static int [,,]dp = new int[100, 100, 100];
    
// Function that count the combination
// of 0's and 1's from the given list
// of String
static int countString(int m, int n,
                String []arr, int i)
{
    // Base Case if count of 0's or 1's
    // becomes negative
    if (m < 0 || n < 0) {
        return int.MinValue;
    }
    
    // If index reaches out of bound
    if (i >= arr.Length) {
        return 0;
    }
    
    // Return the prestored result
    if (dp[m, n, i] != -1) {
        return dp[m, n, i];
    }
    
    // Intialise count of 0's and 1's
    // to 0 for the current state
    int zero = 0, one = 0;
    
    // Calculate the number of 1's and
    // 0's in current String
    foreach (char c in arr[i].ToCharArray()) {
        if (c == '0') {
            zero++;
        }
        else {
            one++;
        }
    }
    
    // Include the current String and
    // recurr for the next iteration
    int x = 1 + countString(m - zero,
                            n - one,
                            arr, i + 1);
    
    // Exclude the current String and
    // recurr for the next iteration
    int y = countString(m, n, arr, i + 1);
    
    // Update the maximum of the above
    // two states to the current dp state
    return dp[m, n, i] = Math.Max(x, y);
}
    
// Driver Code
public static void Main(String[] args)
{
    String []arr = { "10", "0001", "1",
                           "111001", "0" };
    
    // N 0's and M 1's
    int N = 3, M = 5;
    
    // Intialise dp array to -1
    for(int i = 0; i < 100; i++){
        for(int j = 0; j < 100; j++){
            for(int l = 0; l < 100; l++)
            dp[i, j, l] = -1;
        }
    }
    
    // Function call
    Console.Write(countString(M, N, arr, 0));
}
}
  
// This code is contributed by Rajput-Ji
Output:
4

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