使用队列进行层序遍历的同时，将每层的最后一个节点放入结果数组中即可。

class Solution {
public:
    vector<int> rightSideView(TreeNode* root) {
        vector<int> res;
        if (root == nullptr) return res;

        queue<TreeNode*> q;
        q.push(root);
        while (!q.empty())
        {
            int size = q.size();
            while (size -- )
            {
                TreeNode* t = q.front();
                q.pop();
                if (t->left) q.push(t->left);
                if (t->right) q.push(t->right);
                if (size == 0) res.push_back(t->val);   //判断该节点是否是该层最后一个节点
            }            
        }        

        return res;
    }
};