find duplicate subtrees

Author: chen0040

.idea/workspace.xml

@@ -385,3 +385,4 @@ codes may not be optimized
 1. [Validate UTF-8](https://leetcode.com/problems/utf-8-validation/description/)
 1. [Kth Smallest Element in a BST](https://leetcode.com/problems/kth-smallest-element-in-a-bst/description/)
 1. [Serialize and Deserialize BST](https://leetcode.com/problems/serialize-and-deserialize-bst/description/)
+1. [Find Duplicate Subtrees](https://leetcode.com/problems/find-duplicate-subtrees/description/)

README.md

@@ -0,0 +1,63 @@
+package com.github.chen0040.leetcode.day18.medium;
+
+
+import java.util.*;
+
+
+/**
+ * Created by xschen on 13/8/2017.
+ *
+ * link: https://leetcode.com/problems/find-duplicate-subtrees/description/
+ */
+public class FindDuplicateSubtrees {
+    public class TreeNode {
+      int val;
+      TreeNode left;
+      TreeNode right;
+      TreeNode(int x) { val = x; }
+  }
+
+   public class Solution {
+      Map<String, TreeNode> repeated;
+      Set<String> signatures;
+      public List<TreeNode> findDuplicateSubtrees(TreeNode root) {
+         repeated = new HashMap<String, TreeNode>();
+         signatures = new HashSet<String>();
+         preorder(root);
+         return new ArrayList<TreeNode>(repeated.values());
+      }
+
+      private String preorder(TreeNode x) {
+         if(x == null) {
+            return "#";
+         }
+
+         String prefix = "";
+         if(x.left == null && x.right == null) {
+            prefix = "" + x.val;
+         } else {
+            prefix = append(prefix, "" + x.val);
+            prefix = append(prefix, preorder(x.left));
+            prefix = append(prefix, preorder(x.right));
+         }
+
+         //System.out.println(prefix);
+         if(signatures.contains(prefix)) {
+            repeated.put(prefix, x);
+         }
+         signatures.add(prefix);
+
+         return prefix;
+      }
+
+      private String append(String prefix, String val) {
+         if(prefix.equals("")) {
+            prefix = val;
+         } else {
+            prefix = prefix + "," + val;
+         }
+
+         return prefix;
+      }
+   }
+}