add question 648

README.md

@@ -39,6 +39,10 @@
 
 - [q56_合并区间](/src/区间合并/q56_合并区间)
 
+### 字典树（前缀树）
+
+- [q648_单词替换](/src/字典树/q648_单词替换)
+
 ### 字符串操作
 
 - [q6_Z字形变换](/src/字符串操作/q6_Z字形变换)

README_EN.md

@@ -18,7 +18,7 @@
 - [Question 138 : Copy List with Random Pointer](/src/链表操作/q138_复制带随机指针的链表)
 - [Question 206 : Reverse Linked List](/src/链表操作/q206_反转链表)
 
-### Double Pointer Traversal / Sliding Window
+### Two Pointers Traversal / Sliding Window
 
 - [Question 3 : Longest Substring Without Repeating Characters](/src/双指针遍历/q3_无重复字符的最长子串)
 - [Question 11 : Container With Most Water](/src/双指针遍历/q11_盛最多水的容器)
@@ -29,7 +29,7 @@
 - [Question 121 : Best Time to Buy and Sell Stock](/src/双指针遍历/q121_买卖股票的最佳时机)
 - [Question 209 : Minimum Size Subarray Sum](/src/双指针遍历/q209_长度最小的子数组)
 
-### Fast and Slow Pointer Traversal
+### Fast and Slow Pointers Traversal
 
 - [Question 141 : Linked List Cycle](/src/快慢指针遍历/q141_环形链表)
 - [Question 202 : Happy Number](/src/快慢指针遍历/q202_快乐数)
@@ -39,6 +39,10 @@
 
 - [Question 56 : Merge Intervals](/src/区间合并/q56_合并区间)
 
+### Trie
+
+- [Question 648 : Replace Words](/src/字典树/q648_单词替换)
+
 ### String Manipulation
 
 - [Question 6 : ZigZag Conversion](/src/字符串操作/q6_Z字形变换)
@@ -118,7 +122,7 @@
 - [Question 144 : Binary Tree Preorder Traversal](/src/树的遍历/q144_二叉树的前序遍历)
 - [Question 145 : Binary Tree Postorder Traversal](/src/树的遍历/q145_二叉树的后序遍历)
 
-### Binary Search Trees
+### Binary Search Tree
 
 - [Question 98 : Validate Binary Search Tree](/src/二叉搜索树相关/q98_验证二叉搜索树)
 - [Question 450 : Delete Node in a BST](/src/二叉搜索树相关/q450_删除二叉搜索树中的节点)

src/字典树/q648_单词替换/Solution.java

@@ -0,0 +1,49 @@
+package 字典树.q648_单词替换;
+
+import java.util.List;
+
+/**
+ * 构建字典树（前缀树）o(n)
+ */
+class Solution {
+    public String replaceWords(List<String> roots, String sentence) {
+        TrieNode trie = new TrieNode();
+        for (String root : roots) {
+            TrieNode cur = trie;
+            for (char letter : root.toCharArray()) {
+                if (cur.children[letter - 'a'] == null) {
+                    cur.children[letter - 'a'] = new TrieNode();
+                }
+                cur = cur.children[letter - 'a'];
+            }
+            cur.word = root;
+        }
+
+        StringBuilder ans = new StringBuilder();
+
+        for (String word : sentence.split(" ")) {
+            if (ans.length() > 0) {
+                ans.append(" ");
+            }
+
+            TrieNode cur = trie;
+            for (char letter : word.toCharArray()) {
+                if (cur.children[letter - 'a'] == null || cur.word != null) {
+                    break;
+                }
+                cur = cur.children[letter - 'a'];
+            }
+            ans.append(cur.word != null ? cur.word : word);
+        }
+        return ans.toString();
+    }
+}
+
+class TrieNode {
+    TrieNode[] children;
+    String word;
+
+    TrieNode() {
+        children = new TrieNode[26];
+    }
+}