【leetcode】Binary Tree Right Side View（middle）

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]

.

思路：BFS

class Solution {
public:
vector<int> rightSideView(TreeNode *root) {
vector<int> ans;
if(root == NULL) return ans;
queue<TreeNode *> Q;
Q.push(root);

while(!Q.empty())
{
ans.push_back(Q.front()->val);
int pos = Q.size(); //当前层的元素个数
while(pos != 0)
{
if(Q.front()->right != NULL)
Q.push(Q.front()->right);
if(Q.front()->left != NULL)
Q.push(Q.front()->left);
Q.pop();
pos--;
}
}

return ans;
}
};