題目描述

從上往下打印出二叉樹的每個節點，同層節點從左至右列印。

import java.util.ArrayList;
/**
public class TreeNode {
    int val = 0;
    TreeNode left = null;
    TreeNode right = null;

    public TreeNode(int val) {
        this.val = val;

    }

}
*/
public class Solution {
    public ArrayList<Integer> PrintFromTopToBottom(TreeNode root) {
        
    }
}
 

解題思路

• 利用arraylist模擬一個佇列來儲存相應的TreeNode
• 依次判斷左右子樹是否存在，存在就放入佇列中

public class Solution22 {
    /**
     * 利用arraylist模擬一個佇列來儲存相應的TreeNode
     * @param root
     * @return
     */
    public ArrayList<Integer> PrintFromTopToBottom(TreeNode root) {
        ArrayList<Integer> list = new ArrayList<>();
        ArrayList<TreeNode> queue = new ArrayList<>();
        if (root == null) {
            return list;
        }
        queue.add(root);
        while (queue.size() != 0) {
            TreeNode temp = queue.remove(0);
            if (temp.left != null){
                queue.add(temp.left);
            }
            if (temp.right != null) {
                queue.add(temp.right);
            }
            list.add(temp.val);
        }
        return list;
    }

    public class TreeNode {
        int val = 0;
        TreeNode left = null;
        TreeNode right = null;

        public TreeNode(int val) {
            this.val = val;

        }
    }

}