Count Vowels Permutation.java

// Runtime: 41 ms (Top 32.30%) | Memory: 52.8 MB (Top 39.76%)
class Solution {
    private long[][] dp;
    private int mod = (int)1e9 + 7;

    public int countVowelPermutation(int n) {
        dp = new long[6][n+1];
        if(n == 1) return 5;

        for(int i = 0; i < 5; i++)
            dp[i][0] = 1;

        helper(n,'z');
        return (int)dp[5][n];
    }

    private long helper(int n, char vowel)
    {
        long ans = 0;
        if(n == 0) return 1;

        if(vowel == 'z') // we are using z for our convenience just to add Permutations of all Vowels
        {
            ans = (ans + helper(n-1,'a') + helper(n-1,'e') + helper(n-1,'i') + helper(n-1,'o') + helper(n-1,'u'))%mod;
            dp[5][n] = ans;
        }
        // from here as per our assumptions of Vowels we will make calls & store results
        else if(vowel == 'a') // for Nth number we would store Result for "a" in dp[0][n]
        {
            if(dp[0][n] != 0) return dp[0][n];
            ans = (ans + helper(n-1,'e'))%mod;
            dp[0][n] = ans;
        }

        else if(vowel == 'e') // for Nth number we would store Result for "e" in dp[1][n]
        {
            if(dp[1][n] != 0) return dp[1][n];
            ans = (ans + helper(n-1,'a') + helper(n-1,'i'))%mod;
            dp[1][n] = ans;
        }

        else if(vowel == 'i') // for Nth number we would store Result for "i" in dp[2][n]
        {
            if(dp[2][n] != 0) return dp[2][n];
            ans = (ans + helper(n-1,'a') + helper(n-1,'e') + helper(n-1,'o') + helper(n-1,'u'))%mod;
            dp[2][n] = ans;
        }

        else if(vowel == 'o') // for Nth number we would store Result for "o" in dp[3][n]
        {
            if(dp[3][n] != 0) return dp[3][n];
            ans = (ans + helper(n-1,'i') + helper(n-1,'u'))%mod;
            dp[3][n] = ans;
        }

        else // for Nth number we would store Result for "u" in dp[4][n]
        {
            if(dp[4][n] != 0) return dp[4][n];
            ans = (ans + helper(n-1,'a'))%mod;
            dp[4][n] = ans;
        }

        return ans;
    }
}