651. 4 Keys Keyboard

src/main/java/algorithms/curated170/medium/fourkeyskeyboard/FourKeysKeyboardDP.java

@@ -0,0 +1,27 @@
+package algorithms.curated170.medium.fourkeyskeyboard;
+
+public class FourKeysKeyboardDP {
+
+    public int maxA(int N) {
+        int[] best = new int[N + 1];
+
+        for (int press = 1; press <= N; press++) {
+            best[press] = best[press - 1] + 1;
+            if (press > 6) {
+                best[press] = Math.max(best[press], best[press - 4] * 3);
+                best[press] = Math.max(best[press], best[press - 5] * 4);
+                best[press] = Math.max(best[press], best[press - 6] * 5);
+            }
+        }
+        return best[N];
+    }
+
+    public static void main(String[] args) {
+        var solution = new FourKeysKeyboardDP();
+
+        for (int i = 0; i < 50; i++) {
+            System.out.println(solution.maxA(i));
+        }
+
+    }
+}

src/main/java/algorithms/curated170/medium/fourkeyskeyboard/FourKeysKeyboardMath.java

@@ -0,0 +1,23 @@
+package algorithms.curated170.medium.fourkeyskeyboard;
+
+public class FourKeysKeyboardMath {
+
+    final int[] first16Max = new int[] { 0, 1, 2, 3, 4, 5, 6, 9, 12, 16, 20, 27, 36, 48, 64, 81 };
+
+    public int maxA(int N) {
+        int quadrupleTimes = N > 15 ? (N - 11) / 5 : 0;
+        return getQuadrupledCount(N - 5 * quadrupleTimes, quadrupleTimes);
+    }
+
+    private int getQuadrupledCount(int index, int quadrupleTimes) {
+        return first16Max[index] << (2 * quadrupleTimes);
+    }
+
+    public static void main(String[] args) {
+        var solution = new FourKeysKeyboardMath();
+
+        for (int i = 0; i < 50; i++) {
+            System.out.println(solution.maxA(i));
+        }
+    }
+}