feat: add python and java solutions to lcof problem

添加《剑指 Offer》题解：面试题50. 第一个只出现一次的字符

Author: yanglbme

lcof/面试题50. 第一个只出现一次的字符/README.md

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+# [面试题50. 第一个只出现一次的字符](https://leetcode-cn.com/problems/di-yi-ge-zhi-chu-xian-yi-ci-de-zi-fu-lcof/)
+
+## 题目描述
+在字符串 s 中找出第一个只出现一次的字符。如果没有，返回一个单空格。
+
+**示例:**
+
+```
+s = "abaccdeff"
+返回 "b"
+
+s = "" 
+返回 " "
+``` 
+
+**限制：**
+
+- `0 <= s 的长度 <= 50000`
+
+## 解法
+有序字典解决。
+
+### Python3
+```python
+import collections
+
+class Solution:
+    def firstUniqChar(self, s: str) -> str:
+        if s == '':
+            return ' '
+        cache = collections.OrderedDict()
+        for c in s:
+            cache[c] = 1 if cache.get(c) is None else cache[c] + 1
+        for k, v in cache.items():
+            if v == 1:
+                return k
+        return ' '
+```
+
+### Java
+```java
+class Solution {
+    public char firstUniqChar(String s) {
+        if ("".equals(s)) {
+            return ' ';
+        }
+        Map<Character, Integer> map = new LinkedHashMap<>();
+        char[] chars = s.toCharArray();
+        for (char c : chars) {
+            map.put(c, map.get(c) == null ? 1 : 1 + map.get(c));
+        }
+        for (Map.Entry<Character, Integer> entry : map.entrySet()) {
+            if (entry.getValue() == 1) {
+                return entry.getKey();
+            }
+        }
+        return ' ';
+    }
+}
+```
+
+### ...
+```
+
+```

lcof/面试题50. 第一个只出现一次的字符/Solution.java

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+class Solution {
+    public char firstUniqChar(String s) {
+        if ("".equals(s)) {
+            return ' ';
+        }
+        Map<Character, Integer> map = new LinkedHashMap<>();
+        char[] chars = s.toCharArray();
+        for (char c : chars) {
+            map.put(c, map.get(c) == null ? 1 : 1 + map.get(c));
+        }
+        for (Map.Entry<Character, Integer> entry : map.entrySet()) {
+            if (entry.getValue() == 1) {
+                return entry.getKey();
+            }
+        }
+        return ' ';
+    }
+}

lcof/面试题50. 第一个只出现一次的字符/Solution.py

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+import collections
+
+class Solution:
+    def firstUniqChar(self, s: str) -> str:
+        if s == '':
+            return ' '
+        cache = collections.OrderedDict()
+        for c in s:
+            cache[c] = 1 if cache.get(c) is None else cache[c] + 1
+        for k, v in cache.items():
+            if v == 1:
+                return k
+        return ' '