剑指Offer打卡day38—— Acwing 44. 分行从上往下打印二叉树

Posted 2021-05-21 Johnny*

【题目描述】
Acwing 44. 分行从上往下打印二叉树

【思路】

/**
 * 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>> printFromTopToBottom(TreeNode root) {
        
        Queue<TreeNode> q = new LinkedList<>();
        List<List<Integer>> res = new ArrayList<>();
        if( root == null ) return res;
        q.offer(root);
        
        while( !q.isEmpty() ){
            
            int size = q.size();//该层节点数
            List<Integer> sub = new ArrayList<>();
            
            while( size -- > 0){
                //弹出节点
                TreeNode node = q.poll();
                if( node.left != null)  q.offer(node.left);
                if( node.right != null)  q.offer(node.right);
                sub.add(node.val);
            }
            res.add(sub);
        }
        
        return res;
    }
}