Count Sorted Vowel Strings

给 (a, e, i, o, u) 5个元音, 求满足字典序下n个字母的组合.

dp[i][j] = 前i个字符自称的j个元音的总数.

dp[1][i] = [1...5]

dp[i][1] = 1 (a, e, i, o, u)

dp[i][j] = dp[i][j - 1] + dp[i - 1][j]