feat: add python and java solutions to lcof problem

添加《剑指 Offer》题解：面试题64. 求1+2+…+n

Author: yanglbme

lcof/面试题64. 求1+2+…+n/README.md

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+# [面试题64. 求1+2+…+n](https://leetcode-cn.com/problems/qiu-12n-lcof/)
+
+## 题目描述
+求 `1+2+...+n`，要求不能使用乘除法、for、while、if、else、switch、case等关键字及条件判断语句（A?B:C）。
+
+**示例 1：**
+
+```
+输入: n = 3
+输出: 6
+```
+
+**示例 2：**
+
+```
+输入: n = 9
+输出: 45
+```
+
+**限制：**
+
+- `1 <= n <= 10000`
+
+## 解法
+### Python3
+```python
+class Solution:
+    def sumNums(self, n: int) -> int:
+        return n and (n + self.sumNums(n - 1))
+```
+
+### Java
+```java
+class Solution {
+    public int sumNums(int n) {
+        int s = n;
+        boolean t = n > 0 && (s += sumNums(n - 1)) > 0;
+        return s;
+    }
+}
+```
+
+### ...
+```
+
+```

lcof/面试题64. 求1+2+…+n/Solution.java

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+class Solution {
+    public int sumNums(int n) {
+        int s = n;
+        boolean t = n > 0 && (s += sumNums(n - 1)) > 0;
+        return s;
+    }
+}

lcof/面试题64. 求1+2+…+n/Solution.py

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+class Solution:
+    def sumNums(self, n: int) -> int:
+        return n and (n + self.sumNums(n - 1))