LeetCode199.二叉树的右视图

首先需要确定树的高度，再利用递归的思想，先遍历右子树，再遍历左子树。
在遍历过程中，要检查当前层是否访问过，以及当前层高度是否等于树高

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    int getHeight(TreeNode* root)
    {   
        int level = 0;
        TreeNode* cur;
        deque<TreeNode*> queue;
        if (root == NULL) return 0;
        /* 根节点入队 */
        queue.push_back(root);
        /* 遍历 */
        while (!queue.empty())
        {
            int size = queue.size();
            level++;
            for (int i = 0; i < size; i++)
            {
                cur = queue.front();
                queue.pop_front();
                if (cur->left != NULL)
                    queue.push_back(cur->left);
                if (cur->right != NULL)
                    queue.push_back(cur->right);
            }
        }
        return level;
    }
    
    vector<int> rightSideView(TreeNode* root) 
    {   
        vector<int> res;
        int curHeight = 0;
        int curMaxHeight = 0;
        int height = getHeight(root);
        viewFromRight(root, curHeight, curMaxHeight, height, res);
        return res;
    }
    void viewFromRight(TreeNode* root, int& curHeight, int& curMaxHeight, int height, vector<int>& res)
    {   
        if (root == NULL) return;
        curHeight++;
        //根节点
        if (curHeight == 1)
        {
            curMaxHeight = curHeight;
            res.push_back(root->val);
            viewFromRight(root->right, curHeight, curMaxHeight, height, res);
            viewFromRight(root->left, curHeight, curMaxHeight, height, res);
        }
        //已完成遍历
        else if (curMaxHeight == height)
        {
            return;
        }
        //非根节点，且当前层未访问过
        else if (curHeight > curMaxHeight)
        {
            curMaxHeight = curHeight;
            res.push_back(root->val);
            viewFromRight(root->right, curHeight, curMaxHeight, height, res);
            viewFromRight(root->left, curHeight, curMaxHeight, height, res);
        }
        //非根节点，且当前层访问过
        else if (curHeight <= curMaxHeight)
        {
            viewFromRight(root->right, curHeight, curMaxHeight, height, res);
            viewFromRight(root->left, curHeight, curMaxHeight, height, res);
        }
        curHeight--;
    }
};