652 寻找重复的子树

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class Solution {

    ArrayList<TreeNode> res = new ArrayList<>();
    HashMap<String, Integer> map = new HashMap<>();

    public List<TreeNode> findDuplicateSubtrees(TreeNode root) {
        traverse(root);
        return res;
    }

    private String traverse(TreeNode root) {
        if (root == null)
            return "#";
        String left = traverse(root.left);
        String right = traverse(root.right);
        String temp = left + "," + right + "," + String.valueOf(root.val);
        map.put(temp, map.getOrDefault(temp, 0) + 1);
        int freq = map.get(temp);
        if (freq == 2) {
            res.add(root);
        }
        return temp;
    }
}