add NumberOfDigits

realDuYuanChao · realDuYuanChao · commit 22e2e74bf814 · 2020-08-12T22:53:08.000+08:00
diff --git a/Maths/NumberOfDigits.java b/Maths/NumberOfDigits.java
@@ -0,0 +1,62 @@
+package Maths;
+
+/**
+ * Find the number of digits in a number.
+ */
+public class NumberOfDigits {
+    public static void main(String[] args) {
+        int[] numbers = {0, 12, 123, 1234, -12345, 123456, 1234567, 12345678, 123456789};
+        for (int i = 0; i < numbers.length; ++i) {
+            assert numberOfDigits(numbers[i]) == i + 1;
+            assert numberOfDigitsFast(numbers[i]) == i + 1;
+            assert numberOfDigitsFaster(numbers[i]) == i + 1;
+            assert numberOfDigitsRecursion(numbers[i]) == i + 1;
+        }
+    }
+
+    /**
+     * Find the number of digits in a number.
+     *
+     * @param number number to find
+     * @return number of digits of given number
+     */
+    private static int numberOfDigits(int number) {
+        int digits = 0;
+        do {
+            digits++;
+            number /= 10;
+        } while (number != 0);
+        return digits;
+    }
+
+    /**
+     * Find the number of digits in a number fast version.
+     *
+     * @param number number to find
+     * @return number of digits of given number
+     */
+    private static int numberOfDigitsFast(int number) {
+        return number == 0 ? 1 : (int) Math.floor(Math.log10(Math.abs(number)) + 1);
+    }
+
+
+    /**
+     * Find the number of digits in a number faster version.
+     *
+     * @param number number to find
+     * @return number of digits of given number
+     */
+    private static int numberOfDigitsFaster(int number) {
+        return number < 0 ? (-number + "").length() : (number + "").length();
+    }
+
+    /**
+     * Find the number of digits in a number using recursion.
+     *
+     * @param number number to find
+     * @return number of digits of given number
+     */
+    private static int numberOfDigitsRecursion(int number) {
+        return number / 10 == 0 ? 1 : 1 + numberOfDigitsRecursion(number / 10);
+    }
+}
