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<List<Integer>> levelOrder(TreeNode root) {
        List<List<Integer>> res = new ArrayList();
        Queue<TreeNode> queue = new LinkedList();
        if( root == null) return res;
        queue.add(root);

        while(!queue.isEmpty()){
            int size = queue.size();
            List<Integer> 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;
    }
}