LeetCode199. Binary Tree Right Side View

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.
For example:
Given the following binary tree,
1 <—
/ \
2 3 <—
\ \
5 4 <—
You should return [1, 3, 4].
分析：这道题可以用广度优先搜索也可以用深度优先搜索。这里用广度优先方法解决：
如果root为空，则返回空vector。
建立存放TreeNode指针的队列，将root结点入队；
出队root的同时入队root的存在的left和right结点；

按照层序遍历的方式，把每一层的最后一个结点的值存入vector中，最后返回vector。

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