552. Student Attendance Record II
雖然知道這是一道DP題，但是這個dp狀態真的很難找。。。
dp[i] = dp[i].[end with 'P'] + dp[i].[end with 'PL'] + dp[i].[end with 'PLL'] + dp[i].[end with 'LLL']

class Solution(object):
    def checkRecord(self, n):
        if n == 1:
            return 3
        if n == 0:
            return 0
        nums = [1, 1, 2]
        i = 2
        while i < n:
            nums.append((nums[i] + nums[i-1] + nums[i-2])% 1000000007)
            i += 1
        result = (nums[n] + nums[n-1] + nums[n-2]) % 1000000007
        # 這一行表示把A插入到什么位置且A前的一段和A后的一段的變化情況
        for i in range(n):
            result += nums[i+1] * nums[n-i] % 1000000007
            result %= 1000000007
        return result