Leetcode 652.寻找重复的子树

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.

class Solution {
    Map<String, Integer> count;
    List<TreeNode> ans;
    public List<TreeNode> findDuplicateSubtrees(TreeNode root) {
        count = new HashMap();
        ans = new ArrayList();
        collect(root);
        return ans;
    }

    public String collect(TreeNode node) {
        if (node == null) return "#";
        String serial = node.val + "," + collect(node.left) + "," + collect(node.right);
        count.put(serial, count.getOrDefault(serial, 0) + 1);
        if (count.get(serial) == 2)
            ans.add(node);
        return serial;
    }
}