102. 二叉树的层序遍历

方法1：

/**
 * 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<vector<int>> levelOrderBottom(TreeNode* root) 
    {
        if(!root) return {};
        vector<vector<int>> res;
        queue<TreeNode*> qu;
        stack<vector<int>> st;
        qu.push(root);

        while(!qu.empty())
        {
            int num = qu.size();
            vector<int> temp;
            while(num--)
            {
                auto p = qu.front();
                qu.pop();
                temp.push_back(p->val);
                if(p->left) qu.push(p->left);
                if(p->right) qu.push(p->right);
            }
            st.push(std::move(temp));
        }
        while(!st.empty())
        {
            res.push_back(st.top());
            st.pop();
        }
        return res;
    }
};