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

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

Example 1: 

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

      2
     /
    4

    4

class Solution {
    public List<TreeNode> findDuplicateSubtrees(TreeNode root) {
        List<TreeNode> res = new LinkedList<>();
        pos(root,new HashMap<>(),res);
        return res;
        
    }
    public String pos(TreeNode cur,Map<String,Integer> map,List<TreeNode> res){
        if(cur == null)return "#";
        String serial = cur.val + ","+pos(cur.left,map,res)+pos(cur.right,map,res);
        if(map.getOrDefault(serial,0) == 1)
            res.add(cur);
        map.put(serial,map.getOrDefault(serial,0)+1);
        return serial;
    }
}