199. Binary Tree Right Side View

bfs O(n) time O(n) space
bfs逐层遍历每次取最后一个即可

/**
 * 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) {
        if (!root) return {};
        vector<int> res;
        list<TreeNode *> q; // q{{root}};
        q.push_back(root);
        while (!q.empty()) {
            res.push_back(q.back()->val);
            for (int i = q.size(); i > 0; --i) {
                auto p = q.front(); q.pop_front();
                if (p->left) { // 因为要直接access node一定不能把nullptr放进去
                    q.push_back(p->left);
                }
                if (p->right) {
                    q.push_back(p->right);
                }
            }
        }
        return res;
    }
};