力扣102、二叉树的层序遍历
题目：
思路：将第一层节点加入队列，并记录第一层节点个数size，一个一个弹出节点，弹出size个，每弹出一个都将弹出节点的左右孩子加入队列中。
/**
* 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> levelOrder(TreeNode root) {
List> res = new ArrayList();
Queue queue = new LinkedList();
if( root == null) return res;
queue.add(root);
while(!queue.isEmpty()){
int size = queue.size();
List list = new ArrayList();
while(size-- > 0){
TreeNode cur = queue.peek();
queue.poll();
list.add(cur.val);
if(cur.left != null){
queue.add(cur.left);
}
if(cur.right != null){
queue.add(cur.right);
}
}
res.add(list);
}
return res;
}
}
