Problem: Count the Number of Good Subsequences

Medium

30 min

Explore how to identify and count good subsequences in a given string using dynamic programming principles. Learn the definition of good subsequences where each character's frequency is equal and apply modular arithmetic to handle large counts. This lesson helps you build efficient solutions for optimization problems relevant in coding interviews.

Statement

Count and return the number of good subsequences in the given string s. You may return the modulo $10^9 + 7$ of the count.

A subsequence is a sequence formed from another sequence by deleting some or no elements while keeping the order of the remaining elements unchanged.
A good subsequence is a subsequence of a string if it is not empty and the frequency of each character is the same.

Constraints:

$1 \leq$ s.length $\leq 10^4$
s will only contain lowercase English characters.

Problem: Count the Number of Good Subsequences

Medium

30 min

Statement

Count and return the number of good subsequences in the given string s. You may return the modulo $10^9 + 7$ of the count.

A subsequence is a sequence formed from another sequence by deleting some or no elements while keeping the order of the remaining elements unchanged.
A good subsequence is a subsequence of a string if it is not empty and the frequency of each character is the same.

Constraints:

$1 \leq$ s.length $\leq 10^4$
s will only contain lowercase English characters.

Problem: Count the Number of Good Subsequences

Statement

Examples

Understand the problem

Figure it out!

Try it yourself

Problem: Count the Number of Good Subsequences

Statement

Examples

Understand the problem

Figure it out!

Try it yourself