JZ32 Ⅱ从上到下打印二叉树-优快云博客

文章提供了两种使用Java实现二叉树层序遍历的方法。第一种返回层次节点值的列表列表，第二种返回层次节点值的整数数组。方法基于队列进行层次遍历，从根节点开始，逐层添加左右子节点到队列中，直到队列为空。

解析思路：利用层序遍历+队列来实现，一层一层的遍历

/**
 * 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> q = new LinkedList<>();
        q.offer(root);
        while(!q.isEmpty()){
            int size = q.size();
            List<Integer> list = new ArrayList<>();
            for(int i = 0; i < size; i++){
                TreeNode cur = q.poll();
                list.add(cur.val);
                if(cur.left != null){
                    q.offer(cur.left);
                }
                if(cur.right != null){
                    q.offer(cur.right);
                }
            }
            res.add(list);
        }
        return res;

    }
}

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

        }
        int[] ans = new int[res.size()];
        for(int i = 0; i < res.size();i++){
            ans[i] = res.get(i);
        }
        //int[] ans = res.stream().mapToInt(Integer::intValue).toArray();
        return ans;

    }
}