offer2-46

给定一个二叉树的 根节点 root，想象自己站在它的右侧，按照从顶部到底部的顺序，返回从右侧所能看到的节点值。

示例 1:

输入: [1,2,3,null,5,null,4]
输出: [1,3,4]

示例 2:

输入: [1,null,3]
输出: [1,3]

示例 3:

输入: []
输出: []

提示:

• 二叉树的节点个数的范围是 [0,100]
• -100 <= Node.val <= 100

//leetcode submit region begin(Prohibit modification and deletion)
/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public List<Integer> rightSideView(TreeNode root) {
        List<Integer> result = new ArrayList<>();
        Deque<TreeNode> nodeQ = new LinkedList<>();

        if (root == null) {
            return result;
        }

        int nodeNum = 1;
        nodeQ.add(root);

        while (!nodeQ.isEmpty()) {
            result.add(nodeQ.peekLast().val);

            for (int i = 0; i < nodeNum; i++) {
                TreeNode node = nodeQ.poll();

                if (node.left != null) {
                    nodeQ.add(node.left);
                }
                if (node.right != null) {
                    nodeQ.add(node.right);
                }
            }
            nodeNum = nodeQ.size();

        }

        return result;
    }
}
//leetcode submit region end(Prohibit modification and deletion)