[LC 652.寻找重复的子树]

标记一下，暂未弄懂

/**
 * 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 {
    HashMap<String,Integer> record = new HashMap<>();
    LinkedList<TreeNode> res = new LinkedList<>();
    public List<TreeNode> findDuplicateSubtrees(TreeNode root) {
        traverse(root);
        return res;
    }

    String traverse(TreeNode root){
        if (root == null){
            return "#";
        }
        String left = traverse(root.left);
        String right = traverse(root.right);
        String subTree = left+","+right+","+root.val;
        int freq = record.getOrDefault(subTree,0);
        if (freq == 1){
            res.add(root);
        }
        record.put(subTree,freq+1);
        return subTree;
    }
}