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| 1 | +#include <iostream> |
| 2 | +#include <vector> |
| 3 | + |
| 4 | +using namespace std; |
| 5 | + |
| 6 | +const int N = 100 + 5, NO_OF_LETTERS = 4, MOD = 1e9 + 7, T_INDEX = 3; |
| 7 | +long long no_of_strings[N][NO_OF_LETTERS][NO_OF_LETTERS][NO_OF_LETTERS]; |
| 8 | +char letter[NO_OF_LETTERS] = {'A', 'C', 'G', 'T'}; |
| 9 | + |
| 10 | +int ok(int last_1, int last_2, int last_3, int last_4) |
| 11 | +{ |
| 12 | + char l1 = letter[last_1], l2 = letter[last_2], l3 = letter[last_3], l4 = letter[last_4]; |
| 13 | + |
| 14 | + if(l1 == 'A' && l2 == 'G' && l4 == 'C') |
| 15 | + return false; |
| 16 | + if(l1 == 'A' && l3 == 'G' && l4 == 'C') |
| 17 | + return false; |
| 18 | + if(l1 == 'A' && l2 == 'G' && l3 == 'C') |
| 19 | + return false; |
| 20 | + if(l2 == 'A' && l3 == 'G' && l4 == 'C') |
| 21 | + return false; |
| 22 | + if(l1 == 'G' && l2 == 'A' && l3 == 'C') |
| 23 | + return false; |
| 24 | + if(l2 == 'G' && l3 == 'A' && l4 == 'C') |
| 25 | + return false; |
| 26 | + if(l1 == 'A' && l2 == 'C' && l3 == 'G') |
| 27 | + return false; |
| 28 | + if(l2 == 'A' && l3 == 'C' && l4 == 'G') |
| 29 | + return false; |
| 30 | + |
| 31 | + return true; |
| 32 | +} |
| 33 | + |
| 34 | +int main() |
| 35 | +{ |
| 36 | + int length; |
| 37 | + cin >> length; |
| 38 | + |
| 39 | + vector <long long> total(length + 1, 0); |
| 40 | + |
| 41 | + for(int i = 1; i <= length; i++) |
| 42 | + { |
| 43 | + for(int last_1 = 0; last_1 < NO_OF_LETTERS; last_1++) |
| 44 | + { |
| 45 | + for(int last_2 = 0; last_2 < NO_OF_LETTERS; last_2++) |
| 46 | + { |
| 47 | + for(int last_3 = 0; last_3 < NO_OF_LETTERS; last_3++) |
| 48 | + { |
| 49 | + no_of_strings[i][last_1][last_2][last_3] = 0; |
| 50 | + |
| 51 | + for(int last_4 = 0; last_4 < NO_OF_LETTERS; last_4++) |
| 52 | + { |
| 53 | + if(i == 1) |
| 54 | + { |
| 55 | + no_of_strings[i][last_1][T_INDEX][T_INDEX] = 1; |
| 56 | + } |
| 57 | + else if(i == 2) |
| 58 | + { |
| 59 | + no_of_strings[i][last_1][last_2][T_INDEX] = 1; |
| 60 | + } |
| 61 | + else if(i == 3) |
| 62 | + { |
| 63 | + if(ok(T_INDEX, last_3, last_2, last_1)) |
| 64 | + { |
| 65 | + no_of_strings[i][last_1][last_2][last_3] = 1; |
| 66 | + } |
| 67 | + } |
| 68 | + else if(ok(last_4, last_3, last_2, last_1)) |
| 69 | + { |
| 70 | + no_of_strings[i][last_1][last_2][last_3] += no_of_strings[i - 1][last_2][last_3][last_4]; |
| 71 | + } |
| 72 | + } |
| 73 | + |
| 74 | + no_of_strings[i][last_1][last_2][last_3] %= MOD; |
| 75 | + |
| 76 | + total[i] = (total[i] + no_of_strings[i][last_1][last_2][last_3])%MOD; |
| 77 | + } |
| 78 | + } |
| 79 | + } |
| 80 | + } |
| 81 | + |
| 82 | + cout << total[length]; |
| 83 | + return 0; |
| 84 | +} |
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