面试题32 - I. 从上到下打印二叉树

面试题32 - I. 从上到下打印二叉树

比较简单的层序遍历，使用队列遍历每一层并将下一层的节点添加进队列即可。

/**
 * 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> ans = new ArrayList<>();
        Queue<TreeNode> q = new ArrayDeque<>();
        q.offer(root);
        while(!q.isEmpty()) {
            int size = q.size();
            for(int i = 0; i < size; i++) {
                var node = q.poll();
                ans.add(node.val);
                if(node.left != null) q.offer(node.left);
                if(node.right != null) q.offer(node.right);
            }
        }
        int[] ret = new int[ans.size()];
        for(int i = 0; i < ans.size(); i++) ret[i] = ans.get(i);
        return ret;
    }
}