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,null,null,15,7],
3
/
9 20
/
15 7
return its bottom-up level order traversal as:
[
[15,7],
[9,20],
[3]
]
1 /** 2 * Definition for a binary tree node. 3 * struct TreeNode { 4 * int val; 5 * TreeNode *left; 6 * TreeNode *right; 7 * TreeNode(int x) : val(x), left(NULL), right(NULL) {} 8 * }; 9 */ 10 class Solution { 11 public: 12 vector<vector<int>> levelOrderBottom(TreeNode* root) { 13 vector<int> row; 14 vector<vector<int>> v; 15 if(root == NULL) 16 return v; 17 queue<TreeNode*> qu; 18 TreeNode* node; 19 qu.push(root); 20 while(!qu.empty()){ 21 int l = qu.size(); 22 for(int i = 0; i < l; i++){ 23 node = qu.front(); 24 qu.pop(); 25 row.push_back(node -> val); 26 if(node -> left != NULL) 27 qu.push(node -> left); 28 if(node -> right != NULL) 29 qu.push(node -> right); 30 } 31 v.insert(v.begin(), row); 32 row.clear(); 33 } 34 return v; 35 } 36 };