102

/**
 * 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 ArrayList<>();
        if(root == null){
            return res;
        }
        Queue<TreeNode> queue = new LinkedList<TreeNode>();
        queue.add(root);
        int level = 0;
        while(!queue.isEmpty()){
            res.add(new ArrayList<>());
            int levelLength = queue.size();
            
            for(int i = 0; i < levelLength; i++){
                TreeNode node = queue.remove();
                res.get(level).add(node.val);
                
                if(node.left != null){
                    queue.add(node.left);
                }
                if(node.right != null){
                    queue.add(node.right);
                }
            }
            level++;
        }
        return res;
    }
}

/*
二叉树或者多叉树的层次遍历要用队列，将当前子树保存下来，从队列中去除每个节点然后逐层遍历
*/