added problem70

Author: dampdigits

problem70/README.md

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+# 208. Implement Trie (Prefix Tree)
+
+A trie (pronounced as "try") or prefix tree is a tree data structure used to efficiently store and retrieve keys in a dataset of strings. There are various applications of this data structure, such as autocomplete and spellchecker.
+
+Implement the Trie class:
+
+Trie() Initializes the trie object.
+void insert(String word) Inserts the string word into the trie.
+boolean search(String word) Returns true if the string word is in the trie (i.e., was inserted before), and false otherwise.
+boolean startsWith(String prefix) Returns true if there is a previously inserted string word that has the prefix prefix, and false otherwise.
+ 
+
+## Example 1:
+
+**Input**
+["Trie", "insert", "search", "search", "startsWith", "insert", "search"]
+[[], ["apple"], ["apple"], ["app"], ["app"], ["app"], ["app"]]
+
+**Output**
+[null, null, true, false, true, null, true]
+
+Explanation
+Trie trie = new Trie();
+trie.insert("apple");
+trie.search("apple");   // return True
+trie.search("app");     // return False
+trie.startsWith("app"); // return True
+trie.insert("app");
+trie.search("app");     // return True
+ 
+
+## Constraints:
+
+1 <= word.length, prefix.length <= 2000
+word and prefix consist only of lowercase English letters.
+At most 3 * 104 calls in total will be made to insert, search, and startsWith.