LeetCode-199 Binary Tree Right Side View

Input:?[1,2,3,null,5,4]
Output:?[1,4]
Explanation:

   1            <---
 /   2     3         <---
        5     4       <---

/**
 * 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) {
        stack<TreeNode*> str,stran;
        vector<int> res;
        //将根节点入栈
        if(root != NULL)
            str.push(root);
        
        //遍历树的结点
        while(!str.empty()) {
            TreeNode* tmp;
            //保存最靠右的树结点，也就是栈顶元素
            tmp = str.top();
            res.push_back(tmp->val);
            //为了保证栈顶一定为每一层最靠右的结点，需要用另一个栈来进行操作
            //将下一层的结点按从右到左的顺序如另一个栈
            while(!str.empty()) {
                if(tmp->right != NULL)
                    stran.push(tmp->right);
                if(tmp->left != NULL)
                    stran.push(tmp->left);
                str.pop();
                if(!str.empty())
                    tmp = str.top();
            }
            //为保证栈顶为最右结点，将另一个栈中元素按序存入循环使用的栈中
            if(!stran.empty())
                tmp = stran.top();
            while(!stran.empty()) {
                str.push(tmp);
                stran.pop();
                if(!stran.empty())
                    tmp = stran.top();
            }
        }
        
        return res;
    }
};