题目链接:https://leetcode-cn.com/problems/cong-shang-dao-xia-da-yin-er-cha-shu-ii-lcof/
从上到下按层打印二叉树,同一层的节点按从左到右的顺序打印,每一层打印到一行。
解题过程:
二叉树的层次遍历,在每次遍历队列时,设置一个计数,用来将每层的节点值存到不同数组中。
/**
* Definition for a binary tree node.
* public class TreeNode {
* int val;
* TreeNode left;
* TreeNode right;
* TreeNode(int x) { val = x; }
* }
*/
class Solution {
public List<List<Integer>> levelOrder(TreeNode root) {
List<List<Integer>> res = new LinkedList<>();
if (root == null) return res;
Queue<TreeNode> queue = new LinkedList<>();
queue.offer(root);
TreeNode node;
while (! queue.isEmpty()){ //当队列不为空
int num = queue.size();
List<Integer> tmp = new LinkedList<>();
while (num-- != 0){
node = queue.poll();
tmp.add(node.val);
if (node.left != null) queue.offer(node.left);
if (node.right != null) queue.offer(node.right);
}
res.add(tmp);
}
return res;
}
}