週三第九題 不分行從上往下打印二叉樹  Java

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週三第九題 不分行從上往下打印二叉樹 

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 * 思路
 * 需要提前想到使用隊列完成操作
 * a. 將父節點輸出
 * b. 將兒子節點存入隊列中
 * 循環a, b即可
 * 
 * 
 */
class Solution {
    public List<Integer> printFromTopToBottom(TreeNode root) {
        List<Integer> resultList = new ArrayList();
        TreeNode node;
        Queue<TreeNode> nodeQueue = new LinkedList();
        nodeQueue.offer(root);
        while(nodeQueue.peek() != null) {
            node = nodeQueue.poll();
            resultList.add(node.val);
            if(node.left != null) {
                nodeQueue.offer(node.left);
            }
            if(node.right != null) {
                nodeQueue.offer(node.right);
            }
        }
        return resultList;
    }
}