Minimum Cost For Tickets LeetCode solution in Java

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class Solution {
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  public int mincostTickets(int[] days, int[] costs) {
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    HashSet<Integer> set = new HashSet<>();
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    for (int day : days) set.add(day);
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    int n = days[days.length - 1];
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    int[] dp = new int[n + 1];
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    Arrays.fill(dp, Integer.MAX_VALUE);
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    dp[0] = 0;
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    for (int i = 1; i <= n; i++) {
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      if (!set.contains(i)) {
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        dp[i] = dp[i - 1];
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        continue;
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      }
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      int a = dp[Math.max(0, i - 1)] + costs[0];
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      int b = dp[Math.max(0, i - 7)] + costs[1];
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      int c = dp[Math.max(0, i - 30)] + costs[2];
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      dp[i] = Math.min(a, Math.min(b, c));
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    }
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    return dp[n];
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  }
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}