TheAlgorithms · danielUTM · Mar 15, 2021 · Mar 15, 2021 · Mar 15, 2021 · Mar 15, 2021
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+package ProjectEuler;
+
+import java.util.Arrays;
+
+public class Problem19 {
+  public static void main(String[] args) {
+    assert solution() == 171;
+  }
+
+  public static int solution() {
+    int year = 1901;
+    boolean leapYear = false;
+    int totalSundays = 0;
+    int currentDay = 2; // start on a monday
+
+    while (year <= 2000) {
+      leapYear = false;
+      if (year % 4 == 0) {
+        if (year % 100 == 0 && year % 400 == 0) {
+          leapYear = true;
+        } else leapYear = year % 100 != 0 || year % 400 == 0;
+      }
+
+      for (int month = 1; month <= 12; month++) {
+        if (currentDay == 7) {
+          totalSundays++;
+        }
+        if (Arrays.asList(1, 3, 5, 7, 8, 10, 12).contains(month)) {
+          currentDay += 3;
+        } else if (Arrays.asList(4, 6, 9, 11).contains(month)) {
+          currentDay += 2;
+        } else if (leapYear) {
+          currentDay += 1;
+        }
+
+        if (currentDay > 7) {
+          currentDay -= 7;
+        }
+      }
+
+      year++;
+    }
+
+    return totalSundays;
+  }
+}
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+package ProjectEuler;
+
+/**
+ * In the United Kingdom the currency is made up of pound (£) and pence (p). There are eight coins
+ * in general circulation:
+ *
+ * <p>1p, 2p, 5p, 10p, 20p, 50p, £1 (100p), and £2 (200p).
+ *
+ * <p>It is possible to make £2 in the following way:
+ *
+ * <p>1×£1 + 1×50p + 2×20p + 1×5p + 1×2p + 3×1p
+ *
+ * <p>How many different ways can £2 be made using any number of coins?
+ *
+ * <p>link: https://projecteuler.net/problem=31
+ */
+public class Problem31 {
+  public static void main(String[] args) {
+    assert solution() == 73682;
+  }
+
+  public static int solution() {
+    int target = 200;
+    int[] coins = {1, 2, 5, 10, 20, 50, 100, 200};
+    int[] combos = new int[201];
+    combos[0] = 1;
+
+    for (int coin : coins) {
+      for (int i = coin; i <= target; i++) {
+        combos[i] += combos[i - coin];
+      }
+    }
+
+    return combos[200];
+  }
+}