diff --git a/002. Longest Substring Without Repeating Characters/README.md b/002. Longest Substring Without Repeating Characters/README.md
index 224b0b8..2d12d03 100644
--- a/002. Longest Substring Without Repeating Characters/README.md	
+++ b/002. Longest Substring Without Repeating Characters/README.md	
@@ -1,9 +1,19 @@
+# 思路(C++)
+
+此题需要好好读懂题意：**没有重复字符**的最长子字符串的**长度**。
+
+首先，求的只是长度，那么一定有一个 trace 来边记录边比较(max)。
+其次，没有重复字符几乎是唯一条件，那么检查重复显然用 k-v mapping.
+最后，要考虑一次迭代过程中，如何度量这个长度。
+
+----
+
 设 substr 的起点为 start(s), 终点为 last(l). 每一次迭代，记录一张索引表。
 
     abcabcbb
     ^  ^
     s  l
-    
+
 |char|pos|
 |:--:|:-:|
 | a  | 0 |
@@ -25,7 +35,6 @@ cache[s[last]] = last;
 
 注意最终还需要比较一次，返回 `max(ret, s.size() - start)`
 
+## Python
 
-
-
-
+思路与 C++ 完全一致。
\ No newline at end of file
diff --git a/002. Longest Substring Without Repeating Characters/solution.h b/002. Longest Substring Without Repeating Characters/solution.h
index 6c463e6..a614b1e 100644
--- a/002. Longest Substring Without Repeating Characters/solution.h	
+++ b/002. Longest Substring Without Repeating Characters/solution.h	
@@ -8,16 +8,16 @@ using std::max;
 class Solution {
 public:
     int lengthOfLongestSubstring(string s) {
-        size_t ret=0, start=0;
-        unordered_map<char, size_t> cache = {{s.front(), 0}};
-        for (size_t last=1; last < s.size(); ++last) {
-            auto found = cache.find(s[last]);
-            if (found != cache.end() && found->second >= start) {
-                ret = max(ret, last - start);
+        size_t ret = 0, start = 0;
+        unordered_map<char, size_t> trace;
+        for (size_t i = 0; i < s.size(); ++i) {
+            auto found = trace.find(s[i]);
+            if (found != trace.end() && found->second >= start) {
+                ret = max(ret, i - start);
                 start = found->second + 1;
             }
-            cache[s[last]] = last;
+            trace[s[i]] = i;
         }
-        return max(ret, s.size()-start);
+        return max(ret, s.size() - start);
     }
 };
diff --git a/002. Longest Substring Without Repeating Characters/solution.py b/002. Longest Substring Without Repeating Characters/solution.py
new file mode 100644
index 0000000..ee98a44
--- /dev/null
+++ b/002. Longest Substring Without Repeating Characters/solution.py	
@@ -0,0 +1,24 @@
+#!python3
+
+
+class Solution:
+    def lengthOfLongestSubstring(self, s):
+        """
+        :type s: str
+        :rtype: int
+        """
+        rlen = 0
+        start = 0
+        trace = {}
+        for index, ch in enumerate(s):
+            if ch in trace and trace[ch] >= start:
+                rlen = max(rlen, index - start)
+                start = trace[ch] + 1
+            trace[ch] = index
+        return max(rlen, len(s) - start)
+
+
+if __name__ == "__main__":
+    print(Solution().lengthOfLongestSubstring("abcabcbb") == 3)
+    print(Solution().lengthOfLongestSubstring("abba") == 2)
+    print(Solution().lengthOfLongestSubstring("pwwkew") == 3)
diff --git a/README.md b/README.md
index acc608b..f181d0a 100644
--- a/README.md
+++ b/README.md
@@ -3,11 +3,11 @@ LeetCode
 
 LeetCode solutions in C++ 11 and Python3.
 
-|NO.|Title|Solution|Note|Difficulty|
-|---|-----|--------|--------|----------|
-|0|[Two Sum](https://leetcode.com/problems/two-sum)|[C++](000.%20Two%20Sum/solution.h) [Python](000.%20Two%20Sum/solution.py)|[Note](000.%20Two%20Sum)|Easy|
-|1|[Add Two Numbers](https://leetcode.com/problems/add-two-numbers)|[C++](001.%20Add%20Two%20Numbers/solution.h) [Python](001.%20Add%20Two%20Numbers/solution.py)|[Note](001.%20Add%20Two%20Numbers)|Medium|
-|2|[Longest Substring Without Repeating Characters](https://leetcode.com/problems/longest-substring-without-repeating-characters)|[C++](002.%20Longest%20Substring%20Without%20Repeating%20Characters/solution.h) [Python](002.%20Longest%20Substring%20Without%20Repeating%20Characters/solution.py)|[Note](002.%20Longest%20Substring%20Without%20Repeating%20Characters)|Medium|
+|NO.|Title|Solution|Note|Difficulty|Tag|
+|---|-----|--------|----|----------|---|
+|0|[Two Sum](https://leetcode.com/problems/two-sum)|[C++](000.%20Two%20Sum/solution.h) [Python](000.%20Two%20Sum/solution.py)|[Note](000.%20Two%20Sum)|Easy|`Mapping`|
+|1|[Add Two Numbers](https://leetcode.com/problems/add-two-numbers)|[C++](001.%20Add%20Two%20Numbers/solution.h) [Python](001.%20Add%20Two%20Numbers/solution.py)|[Note](001.%20Add%20Two%20Numbers)|Medium|`Linked List`|
+|2|[Longest Substring Without Repeating Characters](https://leetcode.com/problems/longest-substring-without-repeating-characters)|[C++](002.%20Longest%20Substring%20Without%20Repeating%20Characters/solution.h) [Python](002.%20Longest%20Substring%20Without%20Repeating%20Characters/solution.py)|[Note](002.%20Longest%20Substring%20Without%20Repeating%20Characters)|Medium|`Mapping`|
 |3|[Median of Two Sorted Arrays](https://leetcode.com/problems/median-of-two-sorted-arrays)|[C++](003.%20Median%20of%20Two%20Sorted%20Arrays/solution.h) [Python](003.%20Median%20of%20Two%20Sorted%20Arrays/solution.py)|[Note](003.%20Median%20of%20Two%20Sorted%20Arrays)|Hard|
 |4|[Longest Palindromic Substring](https://leetcode.com/problems/longest-palindromic-substring)|[C++](004.%20Longest%20Palindromic%20Substring/solution.h) [Python](004.%20Longest%20Palindromic%20Substring/solution.py)|[Note](004.%20Longest%20Palindromic%20Substring)|Medium|
 |5|[ZigZag Conversion](https://leetcode.com/problems/zigzag-conversion)|[C++](005.%20ZigZag%20Conversion/solution.h) [Python](005.%20ZigZag%20Conversion/solution.py)|[Note](005.%20ZigZag%20Conversion)|Medium|