寻找重复的子树

给定一棵二叉树，返回所有重复的子树。对于同一类的重复子树，你只需要返回其中任意一棵的根结点即可。

两棵树重复是指它们具有相同的结构以及相同的结点值。

 

下面是两个重复的子树：

 

因此，你需要以列表的形式返回上述重复子树的根结点。

 

思路

Intuition

We can serialize each subtree. For example, the tree

   1

  / \

 2   3

    / \

   4   5

can be represented as the serialization 1,2,#,#,3,4,#,#,5,#,#, which is a unique representation of the tree.

Algorithm

Perform a depth-first search, where the recursive function returns the serialization of the tree. At each node, record the result in a map, and analyze the map after to determine duplicate subtrees.

 1 class Solution {
 2     Map<String, Integer> count;
 3     List<TreeNode> ans;
 4     public List<TreeNode> findDuplicateSubtrees(TreeNode root) {
 5         count = new HashMap();
 6         ans = new ArrayList();
 7         collect(root);
 8         return ans;
 9     }
10 
11     public String collect(TreeNode node) {
12         if (node == null) return "#";
13         String serial = node.val + "," + collect(node.left) + "," + collect(node.right);
14         count.put(serial, count.getOrDefault(serial, 0) + 1);
15         if (count.get(serial) == 2)
16             ans.add(node);
17         return serial;
18     }
19 }