Legal string "Dynamic planning"

Strings can consist of a, B, and C three letters, and if any of the next three letters are the same, they are illegal. How many valid strings of length n are available? For example: ABBBCA is illegal, accbcca is legal.

The idea of dynamic planning-do you really want to enumerate?
DP[I][0]: The number of the last two different valid strings of length I
DP[I][1]: The number of the last two bits of the same valid string in length I
Recursion: dp[i][0] = (dp[i-1][0] * 2 + dp[i-1][1] * 2)
DP[I][1] = dp[i-1][0]

Initial
Dp[1][0] = 3, dp[1][1] = 0
Results
Dp[n][0] + dp[n][1]
Space optimization
dp[i][0,1] Only related to dp[i-1][0,1], you can dispense with high-dimensional
Complexity of Time
O (N)

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Legal string "Dynamic planning"