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]