[pull] master from azl397985856:master

README.en.md

@@ -139,6 +139,7 @@ The data structures mainly include:
 - [String Problems](./thinkings/string-problems-en.md)
 - [Sliding Window Technique](./thinkings/slide-window.en.md)
 - [Union Find](./thinkings/union-find.en.md) 🆕
+- [Trie](./thinkings/trie.en.md) 🆕
 
 ### Anki Flashcards
 

assets/drawio/208.implement-trie-prefix-tree.en.drawio

@@ -0,0 +1 @@
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problems/1371.find-the-longest-substring-containing-vowels-in-even-counts.md

@@ -159,11 +159,11 @@ class Solution {
         for (int i = len - 1; i >= 0; i--) {
 
             for (int j = 0; j < len - i; j++) {
-                if (checkValid(preSum, s, i, i + j))
-                    return i + 1
+                if (checkValid(preSum, s, j, i + j))
+                    return i + 1;
             }
         }
-        return 0
+        return 0;
     }
 
 

thinkings/trie.en.md

@@ -0,0 +1,25 @@
+## Trie
+
+When this article is done (2020-07-13), there are 17 LeetCode problems about [Trie (Prefix Tree)](https://leetcode.com/tag/trie/). Among them, 2 problems are easy, 8 are medium, and 7 are hard.
+
+Here we summarize four of them. Once you figure them out, `Trie` should not be a challenge to you anymore. Hope this article is helpful to you.
+
+The main interface of a trie should include the following:
+
+- `insert(word)`: Insert a word
+- `search(word)`: Search for a word
+- `startWith(prefix)`: Search for a word with the given prefix
+
+Among all of the above, `startWith` is one of the most essential methods, which leads to the naming for 'Prefix Tree'. You can start with [208.implement-trie-prefix-tree](https://leetcode.com/problems/implement-trie-prefix-tree) to get yourself familiar with this data structure, and then try to solve other problems.
+
+Here's the graph illustration of a trie: 
+![](../assets/problems/208.implement-trie-prefix-tree-1.en.png)
+
+As the graph shows, each node of the trie would store a character and a boolean `isWord`, which suggests whether the node is the end of a word. There might be some slight differences in the actual implementation, but they are essentially the same.
+
+### Related Problems' Solutions in this Repo (To Be Updated)
+- [0208.implement-trie-prefix-tree](https://github.com/azl397985856/leetcode/blob/b8e8fa5f0554926efa9039495b25ed7fc158372a/problems/208.implement-trie-prefix-tree.md)
+- [0211.add-and-search-word-data-structure-design](https://github.com/azl397985856/leetcode/blob/b0b69f8f11dace3a9040b54532105d42e88e6599/problems/211.add-and-search-word-data-structure-design.md)
+- [0212.word-search-ii](https://github.com/azl397985856/leetcode/blob/b0b69f8f11dace3a9040b54532105d42e88e6599/problems/212.word-search-ii.md)
+- [0472.concatenated-words](https://github.com/azl397985856/leetcode/blob/master/problems/472.concatenated-words.md)
+- [0820.short-encoding-of-words](https://github.com/azl397985856/leetcode/blob/master/problems/820.short-encoding-of-words.md)