# Number of subsequences in a string divisible by n

• Difficulty Level : Hard
• Last Updated : 22 Apr, 2021

Given a string consisting of digits 0-9, count the number of subsequences in it divisible by m.
Examples:

```Input  : str = "1234", n = 4
Output : 4
The subsequences 4, 12, 24 and 124 are
divisible by 4.

Input  : str = "330", n = 6
Output : 4
The subsequences 30, 30, 330 and 0 are
divisible by n.

Input  : str = "676", n = 6
Output : 3
The subsequences 6, 6 and 66```

This problem can be recursively defined. Let remainder of a string with value x be ‘r’ when divided with n. Adding one more character to this string changes its remainder to (r*10 + newdigit) % n. For every new character, we have two choices, either add it in all current subsequences or ignore it. Thus, we have an optimal substructure. Following shows the brute force version of this:

```string str = "330";
int n = 6

// idx is value of current index in str
// rem is current remainder
int count(int idx, int rem)
{
// If last character reached
if (idx == n)
return (rem == 0)? 1 : 0;

int ans = 0;

// we exclude it, thus remainder
// remains the same
ans += count(idx+1, rem);

// we include it and thus new remainder
ans += count(idx+1, (rem*10 + str[idx]-'0')%n);

return ans;
}```

The above recursive solution has overlapping subproblems as shown in below recursion tree.

```          input string = "330"
(0,0) ===> at 0th index with 0 remainder
(exclude 0th /      (include 0th character)
character) /
(1,0)      (1,3) ======> at index 1 with 3 as
(E)/  (I)     /(E)       the current remainder
(2,0)  (2,3)   (2,3)
|-------|
These two subproblems overlap  ```

Thus, we can apply Dynamic Programming. Below is implementation .

## C++

 `// C++ program to count subsequences of a` `// string divisible by n.` `#include` `using` `namespace` `std;`   `// Returns count of subsequences of str` `// divisible by n.` `int` `countDivisibleSubseq(string str, ``int` `n)` `{` `    ``int` `len = str.length();`   `    ``// division by n can leave only n remainder` `    ``// [0..n-1]. dp[i][j] indicates number of` `    ``// subsequences in string [0..i] which leaves` `    ``// remainder j after division by n.` `    ``int` `dp[len][n];` `    ``memset``(dp, 0, ``sizeof``(dp));`   `    ``// Filling value for first digit in str` `    ``dp[(str-``'0'``)%n]++;`   `    ``for` `(``int` `i=1; i

## Java

 `//Java program to count subsequences of a` `// string divisible by n`   `class` `GFG {`   `// Returns count of subsequences of str` `// divisible by n.` `    ``static` `int` `countDivisibleSubseq(String str, ``int` `n) {` `        ``int` `len = str.length();`   `        ``// division by n can leave only n remainder` `        ``// [0..n-1]. dp[i][j] indicates number of` `        ``// subsequences in string [0..i] which leaves` `        ``// remainder j after division by n.` `        ``int` `dp[][] = ``new` `int``[len][n];`   `        ``// Filling value for first digit in str` `        ``dp[``0``][(str.charAt(``0``) - ``'0'``) % n]++;`   `        ``for` `(``int` `i = ``1``; i < len; i++) {` `            ``// start a new subsequence with index i` `            ``dp[i][(str.charAt(i) - ``'0'``) % n]++;`   `            ``for` `(``int` `j = ``0``; j < n; j++) {` `                ``// exclude i'th character from all the` `                ``// current subsequences of string [0...i-1]` `                ``dp[i][j] += dp[i - ``1``][j];`   `                ``// include i'th character in all the current` `                ``// subsequences of string [0...i-1]` `                ``dp[i][(j * ``10` `+ (str.charAt(i) - ``'0'``)) % n] += dp[i - ``1``][j];` `            ``}` `        ``}`   `        ``return` `dp[len - ``1``][``0``];` `    ``}`   `// Driver code` `    ``public` `static` `void` `main(String[] args) {` `        ``String str = ``"1234"``;` `        ``int` `n = ``4``;` `        ``System.out.print(countDivisibleSubseq(str, n));` `    ``}` `}` `// This code is contributed by 29AjayKumar`

## Python 3

 `# Python 3 program to count subsequences ` `# of a string divisible by n.`   `# Returns count of subsequences of ` `# str divisible by n.` `def` `countDivisibleSubseq(``str``, n):` `    ``l ``=` `len``(``str``)`   `    ``# division by n can leave only n remainder` `    ``# [0..n-1]. dp[i][j] indicates number of` `    ``# subsequences in string [0..i] which leaves` `    ``# remainder j after division by n.` `    ``dp ``=` `[[``0` `for` `x ``in` `range``(l)]` `             ``for` `y ``in` `range``(n)]`   `    ``# Filling value for first digit in str` `    ``dp[``0``][(``ord``(``str``[``0``]) ``-` `ord``(``'0'``)) ``%` `n] ``+``=` `1`   `    ``for` `i ``in` `range``(``1``, l):` `    `  `        ``# start a new subsequence with index i` `        ``dp[i][(``ord``(``str``[i]) ``-` `ord``(``'0'``)) ``%` `n] ``+``=` `1`   `        ``for` `j ``in` `range``(n):` `        `  `            ``# exclude i'th character from all the` `            ``# current subsequences of string [0...i-1]` `            ``dp[i][j] ``+``=` `dp[i ``-` `1``][j]`   `            ``# include i'th character in all the current` `            ``# subsequences of string [0...i-1]` `            ``dp[i][(j ``*` `10` `+` `(``ord``(``str``[i]) ``-` `                             ``ord``(``'0'``))) ``%` `n] ``+``=` `dp[i ``-` `1``][j]`   `    ``return` `dp[l ``-` `1``][``0``]`   `# Driver code` `if` `__name__ ``=``=` `"__main__"``:` `    `  `    ``str` `=` `"1234"` `    ``n ``=` `4` `    ``print``(countDivisibleSubseq(``str``, n))`   `# This code is contributed by ita_c`

## C#

 `//C# program to count subsequences of a` `// string divisible by n` ` `  `using` `System;` `class` `GFG {` ` `  `// Returns count of subsequences of str` `// divisible by n.` `    ``static` `int` `countDivisibleSubseq(``string` `str, ``int` `n) {` `        ``int` `len = str.Length;` ` `  `        ``// division by n can leave only n remainder` `        ``// [0..n-1]. dp[i][j] indicates number of` `        ``// subsequences in string [0..i] which leaves` `        ``// remainder j after division by n.` `        ``int``[,] dp = ``new` `int``[len,n];` ` `  `        ``// Filling value for first digit in str` `        ``dp[0,(str - ``'0'``) % n]++;` ` `  `        ``for` `(``int` `i = 1; i < len; i++) {` `            ``// start a new subsequence with index i` `            ``dp[i,(str[i] - ``'0'``) % n]++;` ` `  `            ``for` `(``int` `j = 0; j < n; j++) {` `                ``// exclude i'th character from all the` `                ``// current subsequences of string [0...i-1]` `                ``dp[i,j] += dp[i - 1,j];` ` `  `                ``// include i'th character in all the current` `                ``// subsequences of string [0...i-1]` `                ``dp[i,(j * 10 + (str[i] - ``'0'``)) % n] += dp[i - 1,j];` `            ``}` `        ``}` ` `  `        ``return` `dp[len - 1,0];` `    ``}` ` `  `// Driver code` `    ``public` `static` `void` `Main() {` `        ``String str = ``"1234"``;` `        ``int` `n = 4;` `        ``Console.Write(countDivisibleSubseq(str, n));` `    ``}` `}`

## Javascript

 ``

Output:

`4`

Time Complexity : O(len * n)
Auxiliary Space : O(len * n)
This article is contributed by Ekta Goel. If you like GeeksforGeeks and would like to contribute, you can also write an article using write.geeksforgeeks.org or mail your article to review-team@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.