[LeetCode] 1522. Diameter of N-Ary Tree

Given a root of an N-ary tree, you need to compute the length of the diameter of the tree.

The diameter of an N-ary tree is the length of the longest path between any two nodes in the tree. This path may or may not pass through the root.

(Nary-Tree input serialization is represented in their level order traversal, each group of children is separated by the null value.)

Example 1:

Input: root = [1,null,3,2,4,null,5,6]
Output: 3
Explanation: Diameter is shown in red color.

Example 2:

Input: root = [1,null,2,null,3,4,null,5,null,6]
Output: 4

Example 3:

Input: root = [1,null,2,3,4,5,null,null,6,7,null,8,null,9,10,null,null,11,null,12,null,13,null,null,14]
Output: 7

Constraints:

• The depth of the n-ary tree is less than or equal to 1000.
• The total number of nodes is between [1, 104].

N 叉树的直径。

给定一棵 N 叉树的根节点 root ，计算这棵树的直径长度。

N 叉树的直径指的是树中任意两个节点间路径中 最长 路径的长度。这条路径可能经过根节点，也可能不经过根节点。

（N 叉树的输入序列以层序遍历的形式给出，每组子节点用 null 分隔）

来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/diameter-of-n-ary-tree
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思路是DFS后序遍历。这道题要找的直径长度跟543题类似，也是找树中两条最长的路径和，只不过这道题给的是一个多叉树。既然是多叉树中找两条最长的路径，我们要在过程中一直记录最长的两条子树的长度。往父节点返回的是最长的子树路径 + 1。这道题还是注意最后所求的东西和helper函数返回的东西的区别。

时间O(n)

空间O(n)

Java实现

/*
// Definition for a Node.
class Node {
    public int val;
    public List<Node> children;

    
    public Node() {
        children = new ArrayList<Node>();
    }
    
    public Node(int _val) {
        val = _val;
        children = new ArrayList<Node>();
    }
    
    public Node(int _val,ArrayList<Node> _children) {
        val = _val;
        children = _children;
    }
};
*/

class Solution {
    int res = 0;

    public int diameter(Node root) {
        helper(root);
        return res;
    }

    private int helper(Node root) {
        if (root == null) {
            return 0;
        }
        List<Node> children = root.children;
        int first = 0;
        int second = 0;
        for (Node child : children) {
            int res = helper(child);
            if (res > first) {
                second = first;
                first = res;
            } else if (res > second) {
                second = res;
            }
        }
        res = Math.max(res, first + second);
        return first + 1;
    }
}

 

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1522. Diameter of N-Ary Tree

LeetCode 题目总结