Java LeetCode 102. Sequence traversal of binary tree

Give you a binary tree, please return the node value obtained by traversing it in order. (That is, visit all nodes from left to right layer by layer).

Breadth-first traversal of bfs

/**
 * 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) {
    
    
        if(root==null){
    
    
            return new ArrayList();
        }

        List<List<Integer>> res = new ArrayList();
        
        Deque<TreeNode> que = new LinkedList();

        que.offer(root);

        while(!que.isEmpty()){
    
    
            int size = que.size();
            List<Integer> list = new ArrayList();
            for(int i=0;i<size;i++){
    
    
                if(que.peek().left!=null){
    
    
                    que.offer(que.peek().left);
                }
                if(que.peek().right!=null){
    
    
                    que.offer(que.peek().right);
                }
                list.add(que.poll().val);
            }
            res.add(list);
        }
        return res;

    }
}