LC.P652[寻找重复的子树]

方法一：序列化+DFS

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
/**
 * 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 {

    Map<String, Integer> map = new HashMap<>();
    List<TreeNode> ans = new ArrayList<>();

    public List<TreeNode> findDuplicateSubtrees(TreeNode root) {
        dfs(root);
        return ans;
    }

    // 序列化二叉树，如果某一个结果已经出现过一次，就发现了一类重复子树
    private String dfs(TreeNode node) {
        if (node == null) return " ";
        StringBuilder builder = new StringBuilder();
        builder.append(node.val).append("_");
        builder.append(dfs(node.left)).append(dfs(node.right));
        String key = builder.toString();
        map.merge(key, 1, Integer::sum);
        if (map.get(key) == 2) ans.add(node);
        return key;
    }
}
  • 时间复杂度：$O(n^2)$
  • 空间复杂度：$O(n)$