Algo - problem I

Author: iaramer

python/mipt/mipt_algorithms_course/exam/medium/problem i/dna.py

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+"""
+This problem is in "Normal" group
+
+You are researching different DNAs. DNA is a sequence of nucleotides.
+There are 4 types of nucleotides in DNA: cytosine [C], guanine [G], adenine [A] or thymine [T].
+
+During your previous experiment you found out that you should not consider DNAs which contain at least 3 Adenine
+nucleotides [A] in a row.
+
+Now you want to calculate amount of different DNAs of N nucleotides which you still need to consider.
+(Don't think about complementary sequence, just ignore complementarity rules).
+
+Input format
+Input file contains single integer number 0 < N ≤ 30.
+
+Output format
+Print single integer number: number of DNAs consisting of N nucleotides which doesn't have 3 As in a row.
+"""
+
+DEBUG = True
+
+
+def func(n):
+    dp = [[0] * 2, [0] * 2, [0] * 2]
+    dp[0][1] = 3  # T, G, C
+    dp[1][1] = 1  # A
+    for i in range(2, n + 1):
+        dp[0].append(3 * (dp[0][i - 1] + dp[1][i - 1] + dp[2][i - 1]))
+        dp[1].append(dp[0][i - 1])
+        dp[2].append(dp[1][i - 1])
+    res = dp[0][n] + dp[1][n] + dp[2][n]
+    return res
+
+
+if __name__ == '__main__':
+    if DEBUG:
+        res = func(1)
+        act = 4
+        print(f'{res}, {act}')
+        assert act == res
+
+        res = func(3)
+        act = 63
+        print(f'{res}, {act}')
+        assert act == res
+
+        res = func(10)
+        act = 947808
+        print(f'{res}, {act}')
+        assert act == res
+    else:
+        print(func(int(input())))