leetcode-go/hard/552.student-attendance-record-ii.go

const MOD int = 1000000007

func checkRecord(n int) int {
	// A, L, P
	var dp = [2][3]int{{1, 1, 0}, {1, 0, 0}}
	for i := 1; i < n; i++ {
		var nxt [2][3]int
		for A := 0; A <= 1; A++ {
			for L := 0; L <= 2; L++ {
				nxt[A][0] += dp[A][L] // Append P
				if L > 0 {
					nxt[A][L] += dp[A][L-1] // Append L
				}
				if A > 0 {
					nxt[A][0] += dp[A-1][L] // Append A
				}
				nxt[A][L] %= MOD
			}
		}
		dp = nxt
	}
	sum := 0
	for i := range dp {
		for _, v := range dp[i] {
			sum = (sum + v) % MOD
		}
	}
	return sum
}