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].

【思路】广度遍历，从右到左，vector中只保存树每层最右边的一个节点。

/**
 * 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> result;
        if(NULL==root) return result;
        queue<pair<TreeNode*, int>> q;
        q.push(make_pair(root,0));
        result.push_back(root->val);
        while(!q.empty())
        {
            auto f = q.front();
            q.pop();
            if(f.first->right)
            {
                q.push(make_pair(f.first->right, f.second +1));
            }
            if(f.first->left)
            {
                q.push(make_pair(f.first->left, f.second+1));
            }
            if(!q.empty() && f.second != q.front().second)
            {
                result.push_back(q.front().first->val);
            }
        }
        return result;
    }
};