二叉树的层次遍历

描述

给出一棵二叉树，返回其节点值的层次遍历（逐层从左往右访问）

样例

给一棵二叉树 {3,9,20,#,#,15,7} ：
  3
 / \
9  20
    /  \
   15   7
返回他的分层遍历结果：
[ [3],[9,20],[15,7]

代码实现

/**
 * Definition of TreeNode:
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left, right;
 *     public TreeNode(int val) {
 *         this.val = val;
 *         this.left = this.right = null;
 *     }
 * }
 */
public class Solution {
    /**
     * @param root: The root of binary tree.
     * @return: Level order a list of lists of integer
     */
    public ArrayList<ArrayList<Integer>> levelOrder(TreeNode root) {
        ArrayList<ArrayList<Integer>> result = new ArrayList<>();
        if (root == null) {
            return result;
        }
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);

        while (!queue.isEmpty()) {
            //遍历了一层后重新定义list
            ArrayList<Integer> list = new ArrayList<>();
            int size = queue.size();
           //层级遍历
            for (int i = 0; i < size; i++) {
                TreeNode node = queue.poll();
                list.add(node.val);
                if (node.left != null) {
                    queue.offer(node.left);
                }
                if (node.right != null) {
                    queue.offer(node.right);
                }
            }
            //遍历完一层后加入result
            result.add(list);
        }
        return result;
    }
}