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）。

示例：
二叉树：[3,9,20,null,null,15,7],
3
/ \
9 20
/ \
15 7

返回其层序遍历结果：
[
[3],
[9,20],
[15,7]
]

  1. /**
  2.  * Definition for a binary tree node.
  3.  * public class TreeNode {
  4.  *     int val;
  5.  *     TreeNode left;
  6.  *     TreeNode right;
  7.  *     TreeNode() {}
  8.  *     TreeNode(int val) { this.val = val; }
  9.  *     TreeNode(int val, TreeNode left, TreeNode right) {
  10.  *         this.val = val;
  11.  *         this.left = left;
  12.  *         this.right = right;
  13.  *     }
  14.  * }
  15.  */
  16. class Solution {
  17.    public List<List<Integer>> levelOrder(TreeNode root) {
  18.         List<List<Integer>> ret = new ArrayList<List<Integer>>();
  19.         if (root == null) {
  20.             return ret;
  21.         }
  23.         Queue<TreeNode> queue = new LinkedList<TreeNode>();
  24.         queue.offer(root);
  25.         while (!queue.isEmpty()) {
  26.             List<Integer> level = new ArrayList<Integer>();
  27.             int currentLevelSize = queue.size();
  28.             for (int i = 1; i <= currentLevelSize; ++i) {
  29.                 TreeNode node = queue.poll();
  30.                 level.add(node.val);
  31.                 if (node.left != null) {
  32.                     queue.offer(node.left);
  33.                 }
  34.                 if (node.right != null) {
  35.                     queue.offer(node.right);
  36.                 }
  37.             }
  38.             ret.add(level);
  39.         }
  41.         return ret;
  42.     }
  44. // 作者：LeetCode-Solution
  45. // 链接：https://leetcode-cn.com/problems/binary-tree-level-order-traversal/solution/er-cha-shu-de-ceng-xu-bian-li-by-leetcode-solution/
  46. }