feat: add euler's number

README.md

@@ -7,4 +7,5 @@
 | ModularInv              | 求乘法逆元【扩展欧几里得法】     | -                                                                                                                           |
 | Sqrt10                  | 求 $\sqrt{10}$【连分数】 | [resolve-sqrt-10](https://hatter.ink/static/resource/muboard/?id=resolve-sqrt-10&version=latest&full=1)                     |
 | WallisProduct           | 沃里斯乘积              | [wallis-product](https://hatter.ink/static/resource/muboard/?id=wallis-product&version=latest&full=1)                       |
+| EulerNumber             | 欧拉数                | [eulers-number](https://hatter.ink/static/resource/muboard/?id=eulers-number&version=latest&full=1)                         |
 

src/main/java/me/hatter/math/EulerNumber.java

@@ -0,0 +1,22 @@
+package me.hatter.math;
+
+import java.math.BigDecimal;
+import java.math.RoundingMode;
+
+public class EulerNumber {
+
+    public static void main(String[] args) {
+        // 2.7182818284590452353602874713526624977572470936999595749669676277240766303535475945713821785251664238
+        System.out.println(e());
+    }
+
+    public static BigDecimal e() {
+        BigDecimal e = BigDecimal.valueOf(1);
+        BigDecimal f = BigDecimal.valueOf(1);
+        for (long i = 1; i < 1000; i++) {
+            f = f.multiply(BigDecimal.valueOf(i));
+            e = e.add(BigDecimal.ONE.divide(f, 100, RoundingMode.FLOOR));
+        }
+        return e;
+    }
+}