LeetCode-102-二叉树的遍历

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public List<List<Integer>> levelOrder(TreeNode root){
        if(root == null) {
            return new ArrayList<>();
        }
        List<List<Integer>> res = new ArrayList<List<Integer>>();
        Queue<TreeNode> queue = new LinkedList<TreeNode>();
        queue.add(root);
        while(!queue.isEmpty()) {
            int count = queue.size();
            List<Integer> list = new ArrayList<Integer>();
            while(count > 0) {
                TreeNode node = queue.poll();
                list.add(node.val);
                if(node.left != null) {
                    queue.add(node.left);
                }
                if(node.right != null) {
                    queue.add(node.right);
                }
                count--;
            }
            res.add(list);
        }
        return res;
    }

}