Binary Tree Right Side View

题目：

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.

题目来自leetcode

分析：

目前的解法是O(n)的levelorder traversal，不知道有没有更快的方法。

解法：

class Solution {
public:
    vector<int> rightSideView(TreeNode* root) {
        vector<int> res;
        if (root == NULL) {
            return res;
        }
        queue<TreeNode* > Q;
        Q.push(root);
        while (!Q.empty() ) {
            int size = Q.size();
            for (int i = 0; i < size; i++) {
                TreeNode* temp = Q.front();
                Q.pop();
                if (i == size - 1) {
                    res.push_back(temp->val);
                }
                if (temp->left != NULL) {
                    Q.push(temp->left);
                }
                if (temp->right != NULL) {
                    Q.push(temp->right);
                }
            }
        }
        return res;
    }
};