Count of binary strings of length N with even set bit count and at most K consecutive 1s
Last Updated :
15 Jul, 2025
Given two integers N and K, the task is to find the number of binary strings of length N having an even number of 1's out of which less than K are consecutive.
Examples:
Input: N = 4, K = 2
Output: 4
Explanation:
The possible binary strings are 0000, 0101, 1001, 1010. They all have even number of 1's with less than 2 of them occurring consecutively.
Input: N = 3, K = 2
Output: 2
Explanation:
The possible binary strings are 000, 101. All other strings that is 001, 010, 011, 100, 110, 111 does not meet the criteria.
Approach:
This problem can be solved by Dynamic Programming.
Let us consider a 3D table dp[][][] to store the solution of each subproblem, such that, dp[n][i][s] denotes the number of binary strings of length n having i consecutive 1's and sum of 1's = s. As it is only required to check whether the total number of 1's is even or not we store s % 2. So, dp[n][i][s] can be calculated as follows:
- If we place 0 at the nth position, the number of 1's remain unchanged. Hence, dp[n][i][s] = dp[n - 1][0][s].
- If we place 1 at the nth position, dp[n][i][s] = dp[n - 1][i + 1][(s + 1) % 2] .
- From the above two points the recurrence relation formed is given by:
dp[n][i][s] = dp[n-1][0][s] + dp[n - 1][i + 1][(s + 1)mod 2]
Below is the implementation of the above approach:
C++
// C++ program for the above approach
#include <bits/stdc++.h>
using namespace std;
// Table to store solution
// of each subproblem
int dp[100001][20][2];
// Function to calculate
// the possible binary
// strings
int possibleBinaries(int pos,
int ones,
int sum,
int k)
{
// If number of ones
// is equal to K
if (ones == k)
return 0;
// pos: current position
// Base Case: When n
// length is traversed
if (pos == 0)
// sum: count of 1's
// Return the count
// of 1's obtained
return (sum == 0) ? 1 : 0;
// If the subproblem has already
// been solved
if (dp[pos][ones][sum] != -1)
// Return the answer
return dp[pos][ones][sum];
// Recursive call when current
// position is filled with 1
int ret = possibleBinaries(pos - 1,
ones + 1,
(sum + 1) % 2,
k)
// Recursive call when current
// position is filled with 0
+ possibleBinaries(pos - 1, 0,
sum, k);
// Store the solution
// to this subproblem
dp[pos][ones][sum] = ret;
return dp[pos][ones][sum];
}
// Driver Code
int main()
{
int N = 3;
int K = 2;
// Initialising the
// table with -1
memset(dp, -1, sizeof dp);
cout << possibleBinaries(N, 0, 0, K);
}
// Java program for the above approach
import java.io.*;
class GFG{
// Table to store solution
// of each subproblem
static int [][][]dp = new int[100001][20][2];
// Function to calculate
// the possible binary
// Strings
static int possibleBinaries(int pos, int ones,
int sum, int k)
{
// If number of ones
// is equal to K
if (ones == k)
return 0;
// pos: current position
// Base Case: When n
// length is traversed
if (pos == 0)
// sum: count of 1's
// Return the count
// of 1's obtained
return (sum == 0) ? 1 : 0;
// If the subproblem has already
// been solved
if (dp[pos][ones][sum] != -1)
// Return the answer
return dp[pos][ones][sum];
// Recursive call when current
// position is filled with 1
int ret = possibleBinaries(pos - 1,
ones + 1,
(sum + 1) % 2, k) +
// Recursive call when current
// position is filled with 0
possibleBinaries(pos - 1, 0,
sum, k);
// Store the solution
// to this subproblem
dp[pos][ones][sum] = ret;
return dp[pos][ones][sum];
}
// Driver Code
public static void main(String[] args)
{
int N = 3;
int K = 2;
// Initialising the
// table with -1
for(int i = 0; i < 100001; i++)
{
for(int j = 0; j < 20; j++)
{
for(int l = 0; l < 2; l++)
dp[i][j][l] = -1;
}
}
System.out.print(possibleBinaries(N, 0, 0, K));
}
}
// This code is contributed by Rohit_ranjan
# Python3 program for the above approach
import numpy as np
# Table to store solution
# of each subproblem
dp = np.ones(((100002, 21, 3)))
dp = -1 * dp
# Function to calculate
# the possible binary
# strings
def possibleBinaries(pos, ones, sum, k):
# If number of ones
# is equal to K
if (ones == k):
return 0
# pos: current position
# Base Case: When n
# length is traversed
if (pos == 0):
# sum: count of 1's
# Return the count
# of 1's obtained
return 1 if (sum == 0) else 0
# If the subproblem has already
# been solved
if (dp[pos][ones][sum] != -1):
# Return the answer
return dp[pos][ones][sum]
# Recursive call when current
# position is filled with 1
ret = (possibleBinaries(pos - 1,
ones + 1,
(sum + 1) % 2, k) +
# Recursive call when current
# position is filled with 0
possibleBinaries(pos - 1, 0, sum, k))
# Store the solution
# to this subproblem
dp[pos][ones][sum] = ret
return dp[pos][ones][sum]
# Driver Code
N = 3
K = 2
print(int(possibleBinaries(N, 0, 0, K)))
# This code is contributed by sanjoy_62
// C# program for the above approach
using System;
class GFG{
// Table to store solution
// of each subproblem
static int [,,]dp = new int[100001, 20, 2];
// Function to calculate the
// possible binary Strings
static int possibleBinaries(int pos, int ones,
int sum, int k)
{
// If number of ones
// is equal to K
if (ones == k)
return 0;
// pos: current position
// Base Case: When n
// length is traversed
if (pos == 0)
// sum: count of 1's
// Return the count
// of 1's obtained
return (sum == 0) ? 1 : 0;
// If the subproblem has already
// been solved
if (dp[pos, ones, sum] != -1)
// Return the answer
return dp[pos, ones, sum];
// Recursive call when current
// position is filled with 1
int ret = possibleBinaries(pos - 1,
ones + 1,
(sum + 1) % 2, k) +
// Recursive call when current
// position is filled with 0
possibleBinaries(pos - 1, 0,
sum, k);
// Store the solution
// to this subproblem
dp[pos, ones, sum] = ret;
return dp[pos, ones, sum];
}
// Driver Code
public static void Main(String[] args)
{
int N = 3;
int K = 2;
// Initialising the
// table with -1
for(int i = 0; i < 100001; i++)
{
for(int j = 0; j < 20; j++)
{
for(int l = 0; l < 2; l++)
dp[i, j, l] = -1;
}
}
Console.Write(possibleBinaries(N, 0, 0, K));
}
}
// This code is contributed by Amit Katiyar
<script>
// Javascript program for the above approach
// Table to store solution
// of each subproblem
let dp = new Array(100001).fill(-1).map((t) => new Array(20).fill(-1).map((r) => new Array(2).fill(-1)));
// Function to calculate
// the possible binary
// strings
function possibleBinaries(pos, ones, sum, k)
{
// If number of ones
// is equal to K
if (ones == k)
return 0;
// pos: current position
// Base Case: When n
// length is traversed
if (pos == 0)
// sum: count of 1's
// Return the count
// of 1's obtained
return (sum == 0) ? 1 : 0;
// If the subproblem has already
// been solved
if (dp[pos][ones][sum] != -1)
// Return the answer
return dp[pos][ones][sum];
// Recursive call when current
// position is filled with 1
let ret = possibleBinaries(pos - 1,
ones + 1,
(sum + 1) % 2,
k)
// Recursive call when current
// position is filled with 0
+ possibleBinaries(pos - 1, 0,
sum, k);
// Store the solution
// to this subproblem
dp[pos][ones][sum] = ret;
return dp[pos][ones][sum];
}
// Driver Code
let N = 3;
let K = 2;
// Initialising the
// table with -1
document.write(possibleBinaries(N, 0, 0, K));
// This code is contributed by _saurabh_jaiswal
</script>
Time Complexity: O(2*N*K), where N and K represents the given two integers.
Auxiliary Space: O(100001*20*2), no any other extra space is required, so it is a constant.
Efficient approach : Using DP Tabulation method ( Iterative approach )
The approach to solve this problem is same but DP tabulation(bottom-up) method is better then Dp + memoization(top-down) because memoization method needs extra stack space of recursion calls.
Steps to solve this problem :
- Create a 3D DP table to store the solution of the subproblems.
- Initialize the DP with base cases
- Now Iterate over subproblems to get the value of current problem form previous computation of subproblems stored in DP
- Return the final solution stored in dp[N][0][0].
Implementation :
C++
// C++ code for above approach
#include <bits/stdc++.h>
using namespace std;
// Function to calculate
// the possible binary
// strings
int possibleBinaries(int N, int K) {
// Table to store solution
// of each subproblem
int dp[N+1][K+1][2];
memset(dp, 0, sizeof(dp));
// base case
for(int i=0; i<=K; i++) {
dp[1][i][0] = 1;
dp[1][i][1] = 1;
}
// iterate over subproblems to get the current
// value from previous computation
for(int i=2; i<=N; i++) {
for(int j=0; j<=K; j++) {
for(int k=0; k<=1; k++) {
if(j == K) {
dp[i][j][k] = 0;
} else if(k == 0) {
dp[i][j][k] = dp[i-1][j+1][1];
} else {
dp[i][j][k] = dp[i-1][j+1][0] + dp[i-1][j][1];
}
}
}
}
// return final answer
return dp[N][0][0];
}
// Driver Code
int main() {
int N = 3;
int K = 2;
// Function call
cout << possibleBinaries(N, K);
}
// this code is contributed by bhardwajji
import java.util.Arrays;
public class PossibleBinaries {
// Function to calculate
// the possible binary
// strings
static int possibleBinaries(int N, int K) {
// Table to store solution
// of each subproblem
int[][][] dp = new int[N+1][K+1][2];
for(int i=0; i<=N; i++) {
for(int j=0; j<=K; j++) {
Arrays.fill(dp[i][j], 0);
}
}
// base case
for(int i=0; i<=K; i++) {
dp[1][i][0] = 1;
dp[1][i][1] = 1;
}
// iterate over subproblems to get the current
// value from previous computation
for(int i=2; i<=N; i++) {
for(int j=0; j<=K; j++) {
for(int k=0; k<=1; k++) {
if(j == K) {
dp[i][j][k] = 0;
} else if(k == 0) {
dp[i][j][k] = dp[i-1][j+1][1];
} else {
dp[i][j][k] = dp[i-1][j+1][0] + dp[i-1][j][1];
}
}
}
}
// return final answer
return dp[N][0][0];
}
// Driver Code
public static void main(String[] args) {
int N = 3;
int K = 2;
// Function call
System.out.println(possibleBinaries(N, K));
}
}
# Function to calculate
# the possible binary
# strings
def possibleBinaries(N, K):
# Table to store solution
# of each subproblem
dp = [[[0 for _ in range(2)] for _ in range(K+1)] for _ in range(N+1)]
# base case
for i in range(K+1):
dp[1][i][0] = 1
dp[1][i][1] = 1
# iterate over subproblems to get the current
# value from previous computation
for i in range(2, N+1):
for j in range(K+1):
for k in range(2):
if j == K:
dp[i][j][k] = 0
elif k == 0:
dp[i][j][k] = dp[i-1][j+1][1]
else:
dp[i][j][k] = dp[i-1][j+1][0] + dp[i-1][j][1]
# return final answer
return dp[N][0][0]
# Driver Code
N = 3
K = 2
# Function call
print(possibleBinaries(N, K))
using System;
public class Program {
// Function to calculate
// the possible binary
// strings
public static int PossibleBinaries(int N, int K) {
// Table to store solution
// of each subproblem
int[,,] dp = new int[N+1, K+1, 2];
// base case
for(int i=0; i<=K; i++) {
dp[1, i, 0] = 1;
dp[1, i, 1] = 1;
}
// iterate over subproblems to get the current
// value from previous computation
for(int i=2; i<=N; i++) {
for(int j=0; j<=K; j++) {
for(int k=0; k<=1; k++) {
if(j == K) {
dp[i, j, k] = 0;
} else if(k == 0) {
dp[i, j, k] = dp[i-1, j+1, 1];
} else {
dp[i, j, k] = dp[i-1, j+1, 0] + dp[i-1, j, 1];
}
}
}
}
// return final answer
return dp[N, 0, 0];
}
// Driver Code
public static void Main() {
int N = 3;
int K = 2;
// Function call
Console.WriteLine(PossibleBinaries(N, K));
}
}
// Javascript implementation of given problem
// Function to calculate
// the possible binary
// strings
function possibleBinaries(N, K) {
// Table to store solution
// of each subproblem
var dp = new Array(N+1);
for (var i = 0; i < dp.length; i++) {
dp[i] = new Array(K+1);
for (var j = 0; j < dp[i].length; j++) {
dp[i][j] = new Array(2);
for (var k = 0; k < dp[i][j].length; k++) {
dp[i][j][k] = 0;
}
}
}
// base case
for (var i = 0; i < K+1; i++) {
dp[1][i][0] = 1;
dp[1][i][1] = 1;
}
// iterate over subproblems to get the current
// value from previous computation
for (var i = 2; i < N+1; i++) {
for (var j = 0; j < K+1; j++) {
for (var k = 0; k < 2; k++) {
if (j == K) {
dp[i][j][k] = 0;
} else if (k == 0) {
dp[i][j][k] = dp[i-1][j+1][1];
} else {
dp[i][j][k] = dp[i-1][j+1][0] + dp[i-1][j][1];
}
}
}
}
// return final answer
return dp[N][0][0];
}
// Driver Code
var N = 3;
var K = 2;
// Function call
console.log(possibleBinaries(N, K));
// This code is contributed by Tapesh(tapeshdua420)
Time Complexity : O(N*K*2)
Auxiliary Space : O(N*K*2)
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