Given n, find number of strings of length n, such that string contains at least 2'a', 1'b', 1'c'
if n<=3 there's no such valid string
if n==4, we will have aabc, aacb, abac,.... there are 12 such string, so return 12
if n==5, we have aabca, aabcb,...
n<=10^5
string only contains lowercase letters a-zComments (1)