Create 340._Longest_Substring_with_At_Most_K_Distinct_Characters.md

Author: 0xkookoo

docs/Leetcode_Solutions/Python/340._Longest_Substring_with_At_Most_K_Distinct_Characters.md

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+# 340. Longest Substring with At Most K Distinct Characters
+
+**<font color=red>难度: Hard</font>**
+
+## 刷题内容
+
+> 原题连接
+
+* https://leetcode.com/problems/longest-substring-with-at-most-k-distinct-characters/description/
+
+> 内容描述
+
+```
+
+Given a string, find the length of the longest substring T that contains at most k distinct characters.
+
+Example 1:
+
+Input: s = "eceba", k = 2
+Output: 3
+Explanation: T is "ece" which its length is 3.
+Example 2:
+
+Input: s = "aa", k = 1
+Output: 2
+Explanation: T is "aa" which its length is 2.
+```
+
+## 解题方案
+
+> 思路 1
+******- 时间复杂度: O(N)******- 空间复杂度: O(N)******
+
+
+思路见第159题
+
+```python
+class Solution(object):
+    def lengthOfLongestSubstringKDistinct(self, s, k):
+        """
+        :type s: str
+        :type k: int
+        :rtype: int
+        """
+        maps = {}
+        begin, end, counter, length = 0, 0, 0, 0
+        while end < len(s):
+            maps[s[end]] = maps.get(s[end], 0) + 1
+            if maps[s[end]] == 1:
+                counter += 1
+            end += 1   # end 永远指向下一个待处理的字符
+            while counter > k:
+                maps[s[begin]] -= 1
+                if maps[s[begin]] == 0:
+                    counter -= 1
+                begin += 1
+            length = max(length, end - begin) # 因此这里是```end - begin```而不是```end - begin + 1```
+        return length
+```