0102. 二叉树的层序遍历.java.md

102. 二叉树的层序遍历

给你一个二叉树，请你返回其按 层序遍历 得到的节点值。 （即逐层地，从左到右访问所有节点）。

https://leetcode-cn.com/problems/binary-tree-level-order-traversal/

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/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public List<List<Integer>> levelOrder(TreeNode root) {
        List<List<Integer>> layers = new ArrayList<List<Integer>>();

        if (root == null) {
            return layers;
        }

        Queue<TreeNode> queue = new LinkedList<TreeNode>();
        queue.offer(root);

        while (!queue.isEmpty()) {
            Queue<TreeNode> newQueue = new LinkedList<TreeNode>();
            List<Integer> layer =  new ArrayList<>();
            while (!queue.isEmpty()) {
                TreeNode node = queue.poll();
                layer.add(node.val);
                if (node.left != null) {
                    newQueue.offer(node.left);
                }
                if (node.right != null) {
                    newQueue.offer(node.right);
                }
            };
            layers.add(layer);
            queue = newQueue;
        };

        return layers;
    }
}