LeetCode 199. Binary Tree Right Side View

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:

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

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

解析：从二叉树右侧角度查看二叉树，还是采用队列进行层次遍历二叉树，只不过输出队列尾端的数值，代码如下：

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    vector<int> rightSideView(TreeNode* root) {
        vector<int> result;
        if(root == NULL)
            return result;
        queue<TreeNode*> q;
        q.push(root);
        while(!q.empty())
        {
            int len = q.size();
            result.push_back(q.back()->val);
            for(int i =0 ; i < len; i++)
            {
                TreeNode *temp = q.front();
                q.pop();
                if(temp->left)
                    q.push(temp->left);
                if(temp->right)
                    q.push(temp->right);
            }
        }
        return result;
    }
};