剑指 Offer 32 - II. 从上到下打印二叉树 II

/**
 * 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) {
        //返回类型需要与函数写的相同
        if(root == null) return new LinkedList<>();

        List<List<Integer>> res = new ArrayList<>();
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        
        while(!queue.isEmpty()){
            List<Integer> tmp = new ArrayList<>();
            //关键在这里，不能写(int i = 0; i < queue.size(); i++)
            //因为运行到最后两句，queue.size()回动态变化
            //只有在for循环第一句定义的时候赋值给i，才会避免循环条件改变
            for(int i = queue.size(); i > 0; i--){
                TreeNode 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;
    }
}