Leecode(199) Binary Tree Right Side View

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Description

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.

Example:

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Input: [1,2,3,null,5,null,4]
Output: [1, 3, 4]
Explanation:

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

解法

其实是按层遍历的变形，只要把每层的最后一个节点记录下来就可以了。

具体代码如下：

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/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    List<Integer> res= new ArrayList<>();
    public List<Integer> rightSideView(TreeNode root) {
        if(root == null){
            return res;
        }
        Queue<TreeNode> q = new LinkedList<>();
        q.offer(root);
        while(!q.isEmpty()){
            int size = q.size();
            for(int k = 0; k<size; k++){
                TreeNode tmp = q.poll();
                if(tmp.left!=null){
                    q.offer(tmp.left);
                }
                if(tmp.right!=null){
                    q.offer(tmp.right);
                }
                if(k == size -1 ){
                    res.add(tmp.val);
                }
            }
        }
        return res;
    }
}