1220. Count Vowels Permutation

Description

Given an integer n, your task is to count how many strings of length n can be formed under the following rules:

Each character is a lower case vowel ('a', 'e', 'i', 'o', 'u')
Each vowel 'a' may only be followed by an 'e'.
Each vowel 'e' may only be followed by an 'a' or an 'i'.
Each vowel 'i' may not be followed by another 'i'.
Each vowel 'o' may only be followed by an 'i' or a 'u'.
Each vowel 'u' may only be followed by an 'a'.

Since the answer may be too large, return it modulo 10^9 + 7.

Example 1:

Input: n = 1
Output: 5
Explanation: All possible strings are: "a", "e", "i" , "o" and "u".

Example 2:

Input: n = 2
Output: 10
Explanation: All possible strings are: "ae", "ea", "ei", "ia", "ie", "io", "iu", "oi", "ou" and "ua".

Example 3:

Input: n = 5
Output: 68

Constraints:

1 <= n <= 2 * 10^4

Solutions

Solution 1: Dynamic Programming

Based on the problem description, we can list the possible subsequent vowels for each vowel:

a [e]
e [a|i]
i [a|e|o|u]
o [i|u]
u [a]

From this, we can deduce the possible preceding vowels for each vowel:

[e|i|u] a
[a|i]   e
[e|o]   i
[i] o
[i|o]   u

We define $f[i]$ as the number of strings of the current length ending with the $i$-th vowel. If the length is $1$, then $f[i]=1$.

When the length is greater than $1$, we define $g[i]$ as the number of strings of the current length ending with the $i$-th vowel. Then $g[i]$ can be derived from $f$, that is:

$$ g[i]= \begin{cases} f[1]+f[2]+f[4] & i=0 \ f[0]+f[2] & i=1 \ f[1]+f[3] & i=2 \ f[2] & i=3 \ f[2]+f[3] & i=4 \end{cases} $$

The final answer is $\sum_{i=0}^{4}f[i]$. Note that the answer may be very large, so we need to take the modulus of $10^9+7$.

The time complexity is $O(n)$, and the space complexity is $O(C)$. Here, $n$ is the length of the string, and $C$ is the number of vowels. In this problem, $C=5$.

Python3JavaC++GoTypeScriptJavaScript

class Solution:
    def countVowelPermutation(self, n: int) -> int:
        f = [1] * 5
        mod = 10**9 + 7
        for _ in range(n - 1):
            g = [0] * 5
            g[0] = (f[1] + f[2] + f[4]) % mod
            g[1] = (f[0] + f[2]) % mod
            g[2] = (f[1] + f[3]) % mod
            g[3] = f[2]
            g[4] = (f[2] + f[3]) % mod
            f = g
        return sum(f) % mod

class Solution {
    public int countVowelPermutation(int n) {
        long[] f = new long[5];
        Arrays.fill(f, 1);
        final int mod = (int) 1e9 + 7;
        for (int i = 1; i < n; ++i) {
            long[] g = new long[5];
            g[0] = (f[1] + f[2] + f[4]) % mod;
            g[1] = (f[0] + f[2]) % mod;
            g[2] = (f[1] + f[3]) % mod;
            g[3] = f[2];
            g[4] = (f[2] + f[3]) % mod;
            f = g;
        }
        long ans = 0;
        for (long x : f) {
            ans = (ans + x) % mod;
        }
        return (int) ans;
    }
}

class Solution {
public:
    int countVowelPermutation(int n) {
        using ll = long long;
        vector<ll> f(5, 1);
        const int mod = 1e9 + 7;
        for (int i = 1; i < n; ++i) {
            vector<ll> g(5);
            g[0] = (f[1] + f[2] + f[4]) % mod;
            g[1] = (f[0] + f[2]) % mod;
            g[2] = (f[1] + f[3]) % mod;
            g[3] = f[2];
            g[4] = (f[2] + f[3]) % mod;
            f = move(g);
        }
        return accumulate(f.begin(), f.end(), 0LL) % mod;
    }
};

func countVowelPermutation(n int) (ans int) {
    const mod int = 1e9 + 7
    f := make([]int, 5)
    for i := range f {
        f[i] = 1
    }
    for i := 1; i < n; i++ {
        g := make([]int, 5)
        g[0] = (f[1] + f[2] + f[4]) % mod
        g[1] = (f[0] + f[2]) % mod
        g[2] = (f[1] + f[3]) % mod
        g[3] = f[2] % mod
        g[4] = (f[2] + f[3]) % mod
        f = g
    }
    for _, x := range f {
        ans = (ans + x) % mod
    }
    return
}

function countVowelPermutation(n: number): number {
    const f: number[] = Array(5).fill(1);
    const mod = 1e9 + 7;
    for (let i = 1; i < n; ++i) {
        const g: number[] = Array(5).fill(0);
        g[0] = (f[1] + f[2] + f[4]) % mod;
        g[1] = (f[0] + f[2]) % mod;
        g[2] = (f[1] + f[3]) % mod;
        g[3] = f[2];
        g[4] = (f[2] + f[3]) % mod;
        f.splice(0, 5, ...g);
    }
    return f.reduce((a, b) => (a + b) % mod);
}

/**
 * @param {number} n
 * @return {number}
 */
var countVowelPermutation = function (n) {
    const mod = 1e9 + 7;
    const f = Array(5).fill(1);
    for (let i = 1; i < n; ++i) {
        const g = Array(5).fill(0);
        g[0] = (f[1] + f[2] + f[4]) % mod;
        g[1] = (f[0] + f[2]) % mod;
        g[2] = (f[1] + f[3]) % mod;
        g[3] = f[2];
        g[4] = (f[2] + f[3]) % mod;
        f.splice(0, 5, ...g);
    }
    return f.reduce((a, b) => (a + b) % mod);
};

Solution 2: Matrix Exponentiation to Accelerate Recursion

The time complexity is $O(C^3 \times \log n)$, and the space complexity is $O(C^2)$. Here, $C$ is the number of vowels. In this problem, $C=5$.