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;    }};