Google – Find Duplicated Subtrees


Related: Check if a Binary Tree contains duplicate subtrees of size 2 or more
Google – Find Duplicated Subtrees
Given a binary tree, return all duplicate subtrees.
For example,
        1
       / \
      2   3
     /   / \
    4   2   4
       /
      4
The following are two duplicate subtrees:
      2
     /
    4
and
    4
Therefore, return [ [2,4], [4] ].

给一棵binary tree, 找出这个tree里有多少棵一样的subtree
[Solution]
思路是用string来表示一棵tree,并存在Set里用来检测duplicates。那么某个subtree的表示方法就是
String curr = dfs(root.left) + root.val + dfs(root.right)
但是特别要注意的是,null结点要用特殊符号来表示,比如#,否则没法唯一的确定数的结构。
class Solution {
  int cnt = 0;
  public int duplicatedSubtree(TreeNode root) {
    if (root == null) {
      return 0;
    }
    Set<String> trees = new HashSet<>();
    dfs(root, trees);
    return cnt;
  }
  private String dfs(TreeNode root, Set<String> trees) {
    if (root == null) {
      return "#";
    }
    String left = dfs(root.left, trees);
    String right = dfs(root.right, trees);
    String curr = left + root.val + right;
    if (trees.contains(curr)) {
      cnt++;
    }
    else {
      trees.add(curr);
    }
    return curr;
  }
}
https://discuss.leetcode.com/topic/19/find-duplicate-subtrees/12
Read full article from Google – Find Duplicated Subtrees

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