LeetCode107二叉树的层序遍历II

107. 二叉树的层次遍历 II

例如&＃xff1a;
给定二叉树 [3,9,20,null,null,15,7],

3/ \9 20/ \15 7

返回其自底向上的层次遍历为&＃xff1a;

[[15,7],[9,20],[3]
]

vector<vector<int>> levelOrderBottom(TreeNode* root) {vector<vector<int>> ans;vector<int>cur_lev;if(NULL &＃61;&＃61; root)return ans;queue<TreeNode *>q;q.push(root);while(!q.empty()){int cnt &＃61; q.size();while(cnt){TreeNode* Ntop &＃61; q.front();q.pop();cur_lev.push_back(Ntop->val);if(NULL !&＃61; Ntop->left)q.push(Ntop->left);if(Ntop->right)q.push(Ntop->right);cnt--;}ans.push_back(cur_lev);cur_lev.clear();}reverse(ans.begin(),ans.end());return ans;
}