N-ary Tree Level Order Traversal

Description

Given an n-ary tree, return the level order traversal of its nodes’ values.

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

Example 1:

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

Example 2:

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: [[1],[2,3,4,5],[6,7,8,9,10],[11,12,13],[14]]

Constraints:

• The height of the n-ary tree is less than or equal to 1000
• The total number of nodes is between [0, 104]

Code

Time Complexity: $O(n)$, Space Complexity: $O(1)$

只是 Binary Tree Level Order Traversal 的 generalization 而已。

/*
// Definition for a Node.
class Node {
public:
    int val;
    vector<Node*> children;
 
    Node() {}
 
    Node(int _val) {
        val = _val;
    }
 
    Node(int _val, vector<Node*> _children) {
        val = _val;
        children = _children;
    }
};
*/
 
class Solution {
public:
    vector<vector<int>> levelOrder(Node* root) {
       if(!root) return {};
 
        queue<Node*> q;
        q.push(root);
        vector<vector<int>> res;
        vector<int> level = {};
        while(!q.empty()) {
            int n = q.size();
            level.clear();
            for(int i = 0; i < n; i++) {
                auto node = q.front();
                level.push_back(node->val);
                q.pop();
                for(auto child: node->children)
                    q.push(child);
            }
            res.push_back(level);
        } 
        return res;
    }
};

Source

• N-ary Tree Level Order Traversal - LeetCode