Minimum Cost For Tickets LeetCode solution in C++

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class Solution {
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 public:
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  int f(int i, int j, vector<int> &days, vector<int> &costs, vector<vector<int>> &dp) {
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    if (i == days.size()) return 0;
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    if (days[i] < j) return f(i + 1, j, days, costs, dp);
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    if (dp[i][j] != -1) return dp[i][j];
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    int x = costs[0] + f(i + 1, days[i] + 1, days, costs, dp);
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    int y = costs[1] + f(i + 1, days[i] + 7, days, costs, dp);
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    int z = costs[2] + f(i + 1, days[i] + 30, days, costs, dp);
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    return dp[i][j] = min(x, min(y, z));
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  }
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  int mincostTickets(vector<int> &days, vector<int> &costs) {
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    vector<vector<int>> dp(days.size(), vector<int>(days[days.size() - 1] + 1, -1));
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    return f(0, 1, days, costs, dp);
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  }
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};