Leetcode652 finding duplicate subtrees

Title Description ： Given a binary tree root, Return all duplicate subtrees . For the same kind of repeated subtrees , You just need to return the root node of any tree . If two trees have the same structure and the same node value , Then they are repeated .

analysis ： To find duplicate subtrees , We can first get the subtree of each node , Then compare and judge these subtrees , If repeated, add it to the linked list . How to judge the structure of two subtrees is equal , Can be introduced map Judge , First, store the value of a subtree in map in , Then store each subtree separately map in , If you find that you have deposited before , Then put this node into the linked list , This constant cycle .

Knowledge point ：hashmap Related knowledge

 Map<String,Integer> count = new HashMap();
 count.put(serial,count.getOrDefault(serial,0)+1);
 count.get(serial) == 2;

Linked list knowledge points

    List<TreeNode> ans;