寻找重复的子树
给定一棵二叉树,返回所有重复的子树。对于同一类的重复子树,你只需要返回其中任意一棵的根结点即可。
两棵树重复是指它们具有相同的结构以及相同的结点值。
下面是两个重复的子树:
因此,你需要以列表的形式返回上述重复子树的根结点。
思路
Intuition
We can serialize each subtree. For example, the tree
1
/ \
2 3
/ \
4 5
can be represented as the serialization 1,2,#,#,3,4,#,#,5,#,#, which is a unique representation of the tree.
Algorithm
Perform a depth-first search, where the recursive function returns the serialization of the tree. At each node, record the result in a map, and analyze the map after to determine duplicate subtrees.
class Solution {
Map<String, Integer> count;
List<TreeNode> ans;
public List<TreeNode> findDuplicateSubtrees(TreeNode root) {
count = new HashMap();
ans = new ArrayList();
collect(root);
return ans;
} public String collect(TreeNode node) {
if (node == null) return "#";
String serial = node.val + "," + collect(node.left) + "," + collect(node.right);
count.put(serial, count.getOrDefault(serial, 0) + 1);
if (count.get(serial) == 2)
ans.add(node);
return serial;
}
}