🌗 剑指 Offer 32 - I. 从上到下打印二叉树

难度: 🌗

问题描述

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解法

/**
 * 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) {
        // 思路：
        // 层序 - 辅助队列
        if(root == null) {
            return new int[0];
        }
        ArrayList<Integer> list = new ArrayList<>();
        LinkedList<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        while(!queue.isEmpty()) {
            int len = queue.size(); // 获取当前层节点数
            for(int i = 0; i < len; i ++) {
                TreeNode cur = queue.poll();
                list.add(cur.val);
                if(cur.left != null) {
                    queue.offer(cur.left);
                }
                if(cur.right != null) {
                    queue.offer(cur.right);
                }
            }
        }
        int len = list.size();
        int[] res = new int[len];
        for(int i = 0; i < len; i ++) {
            res[i] = list.get(i);
        }
        return res;
    }
}

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输出