题解 | #从下到上打印二叉树#

/**
 * struct TreeNode {
 *	int val;
 *	struct TreeNode *left;
 *	struct TreeNode *right;
 *	TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 * };
 */
class Solution {
public:
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     * 
     * @param root TreeNode类 
     * @return int整型vector<vector<>>
     */
    vector<vector<int> > levelOrderBottom(TreeNode* root) {
        // write code here
        if(root == nullptr){
            return {};
        }
        
        queue<TreeNode*> Q;
        Q.push(root);
        vector<vector<int>> res;
        
        while(!Q.empty()){
            int cur_size = Q.size();
            vector<int> cur_res;
            for(int i=0; i<cur_size; i++){
                auto node = Q.front();
                Q.pop();
                cur_res.push_back(node->val);
                if(node->left) Q.push(node->left);
                if(node->right) Q.push(node->right);
            }
            res.push_back(cur_res);
        }
        reverse(res.begin(), res.end());
        return res;
    }
};