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;
     }
 }