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LeetCode 199. Binary Tree Right Side View

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题目

原题在此

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.

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

Explanation:

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

解析

使用反向BFS, 先右后左, 在遍历每一层的第一个节点是将此节点值插入vector.

代码

C++

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