题解 | #求二叉树的层序遍历#

/*
 * public class TreeNode {
 *   int val = 0;
 *   TreeNode left = null;
 *   TreeNode right = null;
 * }
 */

public class Solution {
    /**
     * 
     * @param root TreeNode类 
     * @return int整型ArrayList<ArrayList<>>
     */
    public ArrayList<ArrayList<Integer>> levelOrder (TreeNode root) {
        // write code here
        
        ArrayList<ArrayList<Integer>> res=new ArrayList<ArrayList<Integer>>(); 
        if(root==null) return res;
        Queue<TreeNode> queue=new LinkedList<TreeNode>();
        queue.offer(root);
        
        while(!queue.isEmpty()){
            ArrayList<Integer> level=new ArrayList<Integer>();
            int levelNum=queue.size();
            for(int i=0;i<levelNum;i++){
                if(queue.peek().left!=null) queue.offer(queue.peek().left);
                if(queue.peek().right!=null) queue.offer(queue.peek().right);
                level.add(queue.poll().val);
            }
            res.add(level);
        }
        return res;
    }
}