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].

Credits:
Special thanks to @amrsaqr for adding this problem and creating all test cases.

算法：层次遍历的变形，只不过只存储每层最右边的元素。

/** * Definition for binary tree * 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> r;        if(!root){            return r;        }        queue<TreeNode*> q;        q.push(root);        q.push(NULL);//用NULL分开层与层        TreeNode *prev;        do{            TreeNode *n=q.front();            q.pop();            if(n){                prev=n;                if(n->left){                    q.push(n->left);                }                if(n->right){                    q.push(n->right);                }            }else{                r.push_back(prev->val);//NULL前一个元素就是每层的最右边的元素                if(!q.empty()){                    q.push(NULL);                }            }        }while(!q.empty());        return r;    }};