每天一道leetcode-102二叉树的层次遍历

/**
 * 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>> list = new ArrayList<>();
        List<Integer> tempList = new ArrayList<>();
        if(root == null)
            return list;
        Queue<TreeNode> queue = new LinkedList<>();
        queue.add(root);
        int toBePrint = 1;//这一层要打印的节点
        int nextLevelCount = 0;//下一层需要打印节点
        while(queue.isEmpty() == false)//队列不为空
        {
            TreeNode temp = queue.poll();//出队
            tempList.add(temp.val);//把需要的val保存下来
            toBePrint--;//每出队一个，这层要打印的节点数就减少一个
            if(temp.left != null)
            {
                queue.add(temp.left);//入队，先入先出，所以左子树先打印
                nextLevelCount++;//统计下一层节点
            }
            if(temp.right != null)
            {
                queue.add(temp.right);//和上面类似
                nextLevelCount++;
            }
            if(toBePrint == 0)//当这一层节点打印完了
            {
                list.add(new ArrayList<>(tempList));//把这一行的结果保存
                tempList.clear();
                toBePrint = nextLevelCount;//下一层打印的节点，进行赋值
                nextLevelCount = 0;//下下层节点初始值置位0，准备开始计数
            }
        }
        return list;
    }
}