56 lines
1.4 KiB
C++
56 lines
1.4 KiB
C++
#include <bits/stdc++.h>
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typedef long long ll;
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using namespace std;
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ll numDecodings(string s)
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{
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if (s[0] == '0')
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return 0;
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for (int i = 1; i < s.size(); i++)
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{
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if (s[i] == '0' && s[i - 1] == '0')
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return 0;
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}
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// each letter can be a part of the message on their on
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// they can extend the last letter to also form another part of the mesage
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// let dp[i][{0, 1}] be the number of different messages ending at the i(th) char when the i(th) char
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// is not concatenated with i-1(th) char or is.
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// if s[i] is between 1 and 26 then it can be added alone, so dp[i][0] = dp[i - 1][0] + dp[i - 1][1]
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// if s[i] concatenated with s[i - 1] and is between 1 and 26 then they can be added togheter, dp[i][1] = dp[i - 2][0] + dp[i - 2][1]
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// dp[1][0] = 1, dp[1][1] = 0
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int N = s.size();
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vector<vector<ll>> dp(N + 1, vector<ll>(2, 0));
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dp[1][0] = 1, dp[1][1] = 0;
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dp[0][0] = 1;
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for (int i = 2; i <= N; i++)
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{
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char current = s[i - 1], last = s[i - 2];
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string aux = "";
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aux += last;
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aux += current;
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if (current != '0')
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{
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dp[i][0] = dp[i - 1][0] + dp[i - 1][1];
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}
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int code = stoi(aux);
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if (10 <= code && code <= 26)
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{
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dp[i][1] = dp[i - 2][0] + dp[i - 2][1];
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}
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}
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return dp[N][0] + dp[N][1];
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}
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int main()
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{
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int n; cin >> n;
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string code; cin >> code;
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cout << numDecodings(code) << endl;
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return 0;
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} |