Difficulty: Easy
Frequency: N/A
Given a binary tree, return the bottom-up level order traversal of its nodes’ values. (ie, from left to right, level by level from leaf to root).
For example:
Given binary tree {3,9,20,#,#,15,7}
,
3 / \ 9 20 / \ 15 7
return its bottom-up level order traversal as:
[ [15,7], [9,20], [3] ]
/** * Definition for a binary tree node. * struct TreeNode { * int val; * TreeNode *left; * TreeNode *right; * TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */ class Solution { public: vector<vector<int>> levelOrderBottom(TreeNode* root) { vector<vector<int>> results; if (!root) return results; queue<TreeNode*> myQ; myQ.push(root); while (!myQ.empty()) { int q_size = myQ.size(); vector<int> tmp; while (q_size) { TreeNode *cur = myQ.front(); tmp.push_back(cur->val); myQ.pop(); if (cur->left) myQ.push(cur->left); if (cur->right) myQ.push(cur->right); q_size--; } results.push_back(tmp); } return vector<vector<int>> (results.rbegin(), results.rend()); } };