652. 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 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

The following are two duplicate subtrees:

      2
     /
    4

and

    4

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

题目含义：给了一个二叉树，然后需要找出所有重复的子树

    private String serialize(TreeNode node, Map<String, List<TreeNode>> map) {

        if (node == null) return "";
        String s = "<" +serialize(node.left,map) + node.val + serialize(node.right,map) + ">";
        if (!map.containsKey(s)) map.put(s,new ArrayList<>());
        map.get(s).add(node);
        return s;
    }
    
    public List<TreeNode> findDuplicateSubtrees(TreeNode root) {
        List<TreeNode> result = new ArrayList<>();
        if (root == null) return result;
//        将每一个节点的左子节点的值和右结点的值都存储下来，组成一个字符串，作为索引，将对应节点保存到map里。
        Map<String, List<TreeNode>> map = new HashMap<>();
        serialize(root,map);

        for (List<TreeNode>rootGroup :  map.values())
        {
            if (rootGroup.size() > 1) result.add(rootGroup.get(0));
        }
        return result;        
    }