LeetCode-N-ary Tree Level Order Traversal

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Description：
Given an n-ary tree, return the level order traversal of its nodes’ values. (ie, from left to right, level by level).

For example, given a 3-ary tree:
We should return its level order traversal:

[
     [1],
     [3,2,4],
     [5,6]
]

Note:

• The depth of the tree is at most 1000.
• The total number of nodes is at most 5000.

题意：给定一颗N-叉树，返回其层次遍历的节点值；

解法：不管是二叉树还是N叉树，对于层次遍历我们都可以利用队列来实现；算法流程如下：

1. 首先，将根节点入队
2. 当队列不为空时，计算当前队列长度，即当前层节点数
3. 取出当前层所有节点，并且将其孩子节点入队
4. 继续第2步直到队列为空

Java

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

    public Node() {}

    public Node(int _val,List<Node> _children) {
        val = _val;
        children = _children;
    }
};
*/
class Solution {
    public List<List<Integer>> levelOrder(Node root) {
        List<List<Integer>> result = new ArrayList<>();
        if (root == null) return result;
        Deque<Node> nodes = new ArrayDeque<>();
        nodes.addLast(root);
        while (!nodes.isEmpty()) {
            int len = nodes.size();
            List<Integer> temp = new ArrayList<>();
            while (len-- > 0) {
                Node node = nodes.removeFirst();
                temp.add(node.val);
                for (int i = 0; i < node.children.size(); i++) {
                    nodes.addLast(node.children.get(i));
                }
            }
            result.add(temp);
        }
        return result;
    }
}