题解 | #二叉树层序遍历 ii#先层序遍历，再倒置即可

/**
 * struct TreeNode {
 *	int val;
 *	struct TreeNode *left;
 *	struct TreeNode *right;
 * };
 */

class Solution {
public:
    /**
     * 
     * @param root TreeNode类 
     * @return int整型vector<vector<>>
     */
    vector<vector<int> > levelOrderBottom(TreeNode* root) {
        if(root==nullptr) return vector<vector<int>>{};
        vector<vector<int>> arr;
        queue<TreeNode*> q;
        q.push(root);

        while(!q.empty()){
            int n=q.size();
            vector<int> level;
            for(int i=0;i<n;i++){
                TreeNode* node=q.front();
                q.pop();
                level.push_back(node->val);
                if(node->left) q.push(node->left);
                if(node->right) q.push(node->right);
            }
            arr.push_back(level);
        }
        reverse(arr.begin(),arr.end());
        return arr;
    }
};