offer32.java

import java.util.ArrayDeque;
import java.util.ArrayList;
import java.util.Deque;
import java.util.List;

/**
 * [32-I] 从上到下打印二叉树
 * 
 * 题目: 从上到下打印出二叉树的每个节点, 同一层的节点按照从左到右的顺序打印.
 * 
 * 思路: 使用队列来进行层次遍历.
 */

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    /**
     * 时间复杂度: O(n)
     * 空间复杂度: O(n)
     */
    public int[] levelOrder(TreeNode root) {
        if (root == null) return new int[0];
        List<Integer> list = new ArrayList<>();
        Deque<TreeNode> queue = new ArrayDeque<>();
        queue.offer(root);
        while (!queue.isEmpty()) {
            TreeNode cur = queue.poll();
            list.add(cur.val);
            if (cur.left != null) queue.offer(cur.left);
            if (cur.right != null) queue.offer(cur.right);
        }

        return list.stream().mapToInt(Integer::intValue).toArray();
    }
}