652 Find Duplicate Subtrees

652. Find Duplicate Subtrees

1. Question

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

2. Implementation

(1) Post order traversal + HashMap

 public List<TreeNode> findDuplicateSubtrees(TreeNode root) {
        List<TreeNode> res = new ArrayList<>();
        Map<String, Integer> map = new HashMap<>();

        postOrderTraversal(root, map, res);

        return res;
    }

    public String postOrderTraversal(TreeNode node, Map<String, Integer> map, List<TreeNode> res) {
        if (node == null) {
            return "#";
        }

        String leftCode = postOrderTraversal(node.left, map, res);
        String rightCode = postOrderTraversal(node.right, map, res);

        String curCode = node.val + leftCode + rightCode;

        if (map.getOrDefault(curCode, 0) == 1) {
            res.add(node);
        }
        map.put(curCode, map.getOrDefault(curCode, 0) + 1);
        return curCode;
    }

3. Time & Space Complexity

Post order traversal + HashMap: 时间复杂度: O(n), 空间复杂度: O(n)