Leetcode解题: Binary Tree Right Side View (199)

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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].

解法1： BFS, O（N）

这题也是考察BFS算法的一个变形，题意是要求每一层最右边的node。那么我们按层遍历，对于每一层只取第一个（最右边）的node的数值就可以了。
C++

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

Java

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