LeetCode199. Binary Tree Right Side View

一、题目

Given the root of 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.

Example 1:

Input: root = [1,2,3,null,5,null,4]
Output: [1,3,4]
Example 2:

Input: root = [1,null,3]
Output: [1,3]
Example 3:

Input: root = []
Output: []

Constraints:

The number of nodes in the tree is in the range [0, 100].
-100 <= Node.val <= 100

二、题解

类似二叉树的层序遍历，记录每一层中弹出的最后一个数即可

/**
 * 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:
    vector<int> rightSideView(TreeNode* root) {
        vector<int> res;
        queue<TreeNode*> q;
        if(root != nullptr) q.push(root);
        while(!q.empty()){
            int size = q.size();
            while(size--){
                TreeNode* tmp = q.front();
                q.pop();
                if(size == 0) res.push_back(tmp->val);
                if(tmp->left != nullptr) q.push(tmp->left);
                if(tmp->right != nullptr) q.push(tmp->right);
            }
        }
        return res;
    }
};