add trie section

Trie/1032. stream of char/1032.stream-of-characters.cpp

+/*
+ * @lc app=leetcode id=1032 lang=cpp
+ *
+ * [1032] Stream of Characters
+ */
+
+// @lc code=start
+class StreamChecker
+{
+    // 此题对ood也是很好的考察
+    struct TrieNode
+    {
+        bool isEnd;
+        TrieNode *next[26];
+
+        //constructor
+        TrieNode()
+        {
+            isEnd = false;
+            for (int i = 0; i < 26; ++i)
+            {
+                next[i] = nullptr;
+            }
+        }
+    };
+
+    TrieNode *root;
+    string curr;
+
+public:
+    StreamChecker(vector<string> &words)
+    {
+        // construct the trie
+        root = new TrieNode();
+        for (auto &str : words)
+        {
+            addWord(str);
+        }
+    }
+
+    bool query(char letter)
+    {
+        curr.push_back(letter);
+        return find();
+    }
+
+    void addWord(string &str)
+    {
+        // add word from the back to the front
+        TrieNode *ptr = root;
+        for (int i = str.size() - 1; i >= 0; --i)
+        {
+            char ch = str[i];
+            if (ptr->next[ch - 'a'] == nullptr)
+            {
+                ptr->next[ch - 'a'] = new TrieNode();
+            }
+            ptr = ptr->next[ch - 'a'];
+        }
+        ptr->isEnd = true;
+    }
+
+    // find from back to front if curr is in the trie
+    bool find()
+    {
+        TrieNode *ptr = root;
+        int i = curr.size() - 1;
+        while (i >= 0)
+        {
+            char ch = curr[i];
+            if (ptr->next[ch - 'a'])
+            {
+                ptr = ptr->next[ch - 'a'];
+                if (ptr->isEnd)
+                {
+                    return true;
+                }
+                --i;
+            }
+            else
+                break;
+        }
+
+        return false;
+    }
+};
+
+/**
+ * Your StreamChecker object will be instantiated and called as such:
+ * StreamChecker* obj = new StreamChecker(words);
+ * bool param_1 = obj->query(letter);
+ */
+
+/**
+ * Your StreamChecker object will be instantiated and called as such:
+ * StreamChecker* obj = new StreamChecker(words);
+ * bool param_1 = obj->query(letter);
+ */
+// @lc code=end

Trie/1032. stream of char/Readme.md

+# 1032. Stream of char
+
+前缀树的题目的代码量很大，但是思路很好想，所以熟练度很重要
+
+### 核心思想:
+
+每次query一个char，从那个字符开始往前看，所以建树时也要从后往前:
+
+```cpp
+        void addWord(string &str)
+            {
+                // add word from the back to the front
+                TrieNode *ptr = root;
+                for (int i = str.size() - 1; i >= 0; --i)
+                {
+                    char ch = str[i];
+                    if (ptr->next[ch - 'a'] == nullptr)
+                    {
+                        ptr->next[ch - 'a'] = new TrieNode();
+                    }
+                    ptr = ptr->next[ch - 'a'];
+                }
+                ptr->isEnd = true;
+            }
+```
+
+此题其他部分就走流程，先要implement一个trieNode，在把string放入即可：
+
+```cpp 
+        struct TrieNode
+        {
+            bool isEnd;
+            TrieNode *next[26];
+
+            //constructor
+            TrieNode()
+            {
+                isEnd = false;
+                for (int i = 0; i < 26; ++i)
+                {
+                    next[i] = nullptr;
+                }
+            }
+        };
+```
+