Trie

Author: JoshuaJiangFD

src/main/java/joshua/leetcode/design/Trie.java

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+package joshua.leetcode.design;
+
+/**
+ * 208. Implement Trie (Prefix Tree) <br/>
+ * 字典树，或称前缀树 是一种快速查找前缀匹配的数据结构，例如给定一个word list，构建一个前缀树后
+ * 可以快速的查找某个word是否在该字典中。
+ * <p/>
+ * <a href="https://leetcode.com/problems/implement-trie-prefix-tree/">leetcode link</a>
+ *
+ * @author Joshua.Jiang on 2016/5/30.
+ */
+public class Trie {
+
+    private TrieNode root;
+
+
+    public Trie() {
+        root = new TrieNode();
+    }
+
+    // Inserts a word into the trie.
+    public void insert(String word) {
+        TrieNode parentNode = root;
+        char[] chars = word.toCharArray();
+        for (int i = 0; i < chars.length; i++) {
+            char ch = chars[i];
+            if(parentNode.nodes[ch - 'a'] == null) {
+                parentNode.nodes[ch - 'a'] = new TrieNode();
+                if (i < chars.length-1) {
+                    parentNode.nodes[ch - 'a'].isLeafNode = false;
+                }
+            }
+            if (i == chars.length - 1) {
+                parentNode.nodes[ch - 'a'].isLeafNode = true;
+            } else {
+                parentNode = parentNode.nodes[ch - 'a'];
+            }
+        }
+    }
+
+    // Returns if the word is in the trie.
+    public boolean search(String word) {
+        TrieNode parentNode = root;
+        for(char ch : word.toCharArray()) {
+            if (parentNode.nodes[ch - 'a'] == null) {
+                return false;
+            }
+            parentNode = parentNode.nodes[ch - 'a'];
+        }
+        return parentNode.isLeafNode;
+    }
+
+    // Returns if there is any word in the trie
+    // that starts with the given prefix.
+    public boolean startsWith(String prefix) {
+        TrieNode parentNode = root;
+        for(char ch : prefix.toCharArray()) {
+            if (parentNode.nodes[ch - 'a'] == null) {
+                return false;
+            }
+            parentNode = parentNode.nodes[ch - 'a'];
+        }
+        return true;
+    }
+}
+
+class TrieNode {
+    /**
+     * nodes的下标表示一个字符,例如: nodes[0]不为空，表示存在字符为'a'的子节点
+     */
+    TrieNode[] nodes = new TrieNode[26];
+    boolean isLeafNode = true;
+
+    public TrieNode() {
+    }
+}

src/test/java/joshua/leetcode/design/TrieTest.java

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+package joshua.leetcode.design;
+
+import org.junit.Test;
+
+import static org.junit.Assert.*;
+
+public class TrieTest {
+
+    @Test
+    public void testTrie() {
+        Trie trie = new Trie();
+//        trie.insert("somestring");
+//        assertTrue(trie.startsWith("some"));
+//        assertFalse(trie.startsWith("omes"));
+//        assertTrue(trie.search("somestring"));
+        trie.insert("app");
+        trie.insert("apple");
+        assertFalse(trie.search("apps"));
+        assertTrue(trie.search("app"));
+    }
+
+    @Test
+    public void testTrie2() {
+        Trie trie = new Trie();
+        trie.insert("abc");
+        assertTrue(trie.search("abc"));
+        assertFalse(trie.search("ab"));
+        trie.insert("ab");
+        assertTrue(trie.search("ab"));
+        trie.insert("ab");
+        assertTrue(trie.search("ab"));
+    }
+}