Find Duplicate Subtrees

Given a binary tree, return all duplicate subtrees. For each kind of duplicate subtrees, you only need to return the root node of any one of them.

Two trees are duplicate if they have the same structure with same node values.

Example 1:

        1
       / \
      2   3
     /   / \
    4   2   4
       /
      4

The following are two duplicate subtrees:

      2
     /
    4

and

    4

Therefore, you need to return above trees' root in the form of a list.

思路：这题解法比较巧妙的是，把node serilize一下，用string来表示node下面的tree，这样如果string相同，就代表node tree一样, 然后每次覆盖node，也就避免了重复的问题；

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public List<TreeNode> findDuplicateSubtrees(TreeNode root) {
        List<TreeNode> list = new ArrayList<>();
        HashMap<String, Integer> hashmap = new HashMap<>();
        dfs(root, hashmap, list);
        return list;
    }
    
    private String dfs(TreeNode root, HashMap<String, Integer> hashmap, List<TreeNode> list) {
        if(root == null) {
            return "#";
        }
        String nodestr = root.val + " "
                        + dfs(root.left, hashmap, list) + " "
                        + dfs(root.right, hashmap, list);
        hashmap.put(nodestr, hashmap.getOrDefault(nodestr, 0) + 1);
        if(hashmap.get(nodestr) == 2) {
            list.add(root);
        }
        return nodestr;
    }
}