17. Count Sorted Vowel Strings

class Solution {
    char[] ch=new char[]{'a','e','i','o','u'};
    public int countVowelStrings(int n) {
        int ans=0;
        for(char c:ch){
            ans+=count(n-1,c);
        }
        
        return ans;
    }
    
    int count(int length,char first){
        if(length==0){
            return 1;
        }
        int ans=0;
        for(char c:ch){
            if(first>=c){
                ans+=count(length-1,c);
            }
        }
        
        return ans;
    }
}

class Solution {
    public int countVowelStrings(int n) {
        int[][] dp=new int[n+1][6];
        
        for(int i=1;i<=n;i++){
            for(int j=1;j<=5;j++){
                dp[i][j]=dp[i][j-1]+(i>1?dp[i-1][j]:1);
            }
        }
        
        return dp[n][5];
    }
}

class Solution {
    public int countVowelStrings(int n) {
        return (n+4)*(n+3)*(n+2)*(n+1)/24;
    }
}