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2842. Count K-Subsequences of a String With Maximum Beauty

Description

You are given a string s and an integer k.

A k-subsequence is a subsequence of s, having length k, and all its characters are unique, i.e., every character occurs once.

Let f(c) denote the number of times the character c occurs in s.

The beauty of a k-subsequence is the sum of f(c) for every character c in the k-subsequence.

For example, consider s = "abbbdd" and k = 2:

  • f('a') = 1, f('b') = 3, f('d') = 2
  • Some k-subsequences of s are:
    • "abbbdd" -> "ab" having a beauty of f('a') + f('b') = 4
    • "abbbdd" -> "ad" having a beauty of f('a') + f('d') = 3
    • "abbbdd" -> "bd" having a beauty of f('b') + f('d') = 5

Return an integer denoting the number of k-subsequences whose beauty is the maximum among all k-subsequences. Since the answer may be too large, return it modulo 109 + 7.

A subsequence of a string is a new string formed from the original string by deleting some (possibly none) of the characters without disturbing the relative positions of the remaining characters.

Notes

  • f(c) is the number of times a character c occurs in s, not a k-subsequence.
  • Two k-subsequences are considered different if one is formed by an index that is not present in the other. So, two k-subsequences may form the same string.

 

Example 1:

Input: s = "bcca", k = 2
Output: 4
Explanation: From s we have f('a') = 1, f('b') = 1, and f('c') = 2.
The k-subsequences of s are: 
bcca having a beauty of f('b') + f('c') = 3 
bcca having a beauty of f('b') + f('c') = 3 
bcca having a beauty of f('b') + f('a') = 2 
bcca having a beauty of f('c') + f('a') = 3
bcca having a beauty of f('c') + f('a') = 3 
There are 4 k-subsequences that have the maximum beauty, 3. 
Hence, the answer is 4.

Example 2:

Input: s = "abbcd", k = 4
Output: 2
Explanation: From s we have f('a') = 1, f('b') = 2, f('c') = 1, and f('d') = 1. 
The k-subsequences of s are: 
abbcd having a beauty of f('a') + f('b') + f('c') + f('d') = 5
abbcd having a beauty of f('a') + f('b') + f('c') + f('d') = 5 
There are 2 k-subsequences that have the maximum beauty, 5. 
Hence, the answer is 2.

 

Constraints:

  • 1 <= s.length <= 2 * 105
  • 1 <= k <= s.length
  • s consists only of lowercase English letters.

Solutions

Solution 1: Greedy + Combinatorial Mathematics

First, we use a hash table $f$ to count the occurrence of each character in the string $s$, i.e., $f[c]$ represents the number of times character $c$ appears in the string $s$.

Since a $k$-subsequence is a subsequence of length $k$ in the string $s$ with unique characters, if the number of different characters in $f$ is less than $k$, then there is no $k$-subsequence, and we can directly return $0$.

Otherwise, to maximize the beauty value of the $k$-subsequence, we need to make characters with high beauty values appear as much as possible in the $k$-subsequence. Therefore, we can sort the values in $f$ in reverse order to get an array $vs$.

We denote the occurrence of the $k$th character in the array $vs$ as $val$, and there are $x$ characters with an occurrence of $val$.

Then we first find out the characters with occurrences greater than $val$, multiply the occurrences of each character to get the initial answer $ans$, and update the remaining number of characters to be selected to $k$. We need to select $k$ characters from $x$ characters, so the answer needs to be multiplied by the combination number $C_x^k$, and finally multiplied by $val^k$, i.e., $ans = ans \times C_x^k \times val^k$.

Note that we need to use fast power and modulo operations here.

The time complexity is $O(n)$, and the space complexity is $O(|\Sigma|)$. Here, $n$ is the length of the string, and $\Sigma$ is the character set. In this problem, the character set is lowercase letters, so $|\Sigma| = 26$.

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class Solution:
    def countKSubsequencesWithMaxBeauty(self, s: str, k: int) -> int:
        f = Counter(s)
        if len(f) < k:
            return 0
        mod = 10**9 + 7
        vs = sorted(f.values(), reverse=True)
        val = vs[k - 1]
        x = vs.count(val)
        ans = 1
        for v in vs:
            if v == val:
                break
            k -= 1
            ans = ans * v % mod
        ans = ans * comb(x, k) * pow(val, k, mod) % mod
        return ans

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class Solution {
    private final int mod = (int) 1e9 + 7;

    public int countKSubsequencesWithMaxBeauty(String s, int k) {
        int[] f = new int[26];
        int n = s.length();
        int cnt = 0;
        for (int i = 0; i < n; ++i) {
            if (++f[s.charAt(i) - 'a'] == 1) {
                ++cnt;
            }
        }
        if (cnt < k) {
            return 0;
        }
        Integer[] vs = new Integer[cnt];
        for (int i = 0, j = 0; i < 26; ++i) {
            if (f[i] > 0) {
                vs[j++] = f[i];
            }
        }
        Arrays.sort(vs, (a, b) -> b - a);
        long ans = 1;
        int val = vs[k - 1];
        int x = 0;
        for (int v : vs) {
            if (v == val) {
                ++x;
            }
        }
        for (int v : vs) {
            if (v == val) {
                break;
            }
            --k;
            ans = ans * v % mod;