199. Binary Tree Right Side View -----层序遍历

Given a binary tree, imagine yourself standing on the right side of it, return the values of the nodes you can see ordered from top to bottom.

For example:
Given the following binary tree,

   1            <---
 /   
2     3         <---
      
  5     4       <---

You should return [1, 3, 4].

其实就是层序遍历 的每层的最后一个元素！！！！

class Solution {
    
    public List<Integer> rightSideView(TreeNode root) {
        List<Integer> res = new ArrayList<Integer>();
        Queue<TreeNode> queue = new LinkedList<TreeNode>();
        if(root==null) return res;
        queue.offer(root);
        int level_num = 1;
        while (!queue.isEmpty()) {
            level_num = queue.size();
            for(int i = 0; i < level_num; i++){
                TreeNode node = queue.poll();
                if(i==level_num-1)
                    res.add(node.val);
                if(node.left != null) queue.offer(node.left);
                if(node.right != null) queue.offer(node.right);
                
            }
        }
        return res;
    }
}