1     vector<vector<int> > levelOrderBottom(TreeNode *root) {
 2         vector<vector<int> > levels;
 3         if(!root) return levels;
 4         queue<TreeNode*> toVisit;
 5         toVisit.push(root);
 6         int numLevelNodes = 1;
 7         while(!toVisit.empty()) {
 8             vector<int> level;
 9             for(int i = 0; i < numLevelNodes; i++) {
10                 TreeNode *node = toVisit.front();
11                 toVisit.pop();
12                 level.push_back(node -> val);
13                 if(node -> left) toVisit.push(node -> left);
14                 if(node -> right) toVisit.push(node -> right);
15             }
16             if (!level.empty()) levels.push_back(level);
17             numLevelNodes = toVisit.size();
18         }
19         reverse(levels.begin(), levels.end());
20         return levels;
21     }

 1     vector<vector<int>> levelOrderBottom(TreeNode *root) {
 2         vector<vector<int>> levels;
 3         if(!root) return levels;
 4         int curLevel = 1;
 5         bool nextLevel = true;
 6         while(nextLevel) {
 7             vector<int> level;
 8             nextLevel = false;
 9             levelTraverse(root, curLevel, nextLevel, level);
10             levels.push_back(level);
11             curLevel++;
12         }
13         reverse(levels.begin(), levels.end());
14         return levels;
15     }
16     void levelTraverse(TreeNode* node, int curLevel, bool& nextLevel, vector<int>& level) {
17         if(!node) return;
18         if(curLevel == 1) {
19             level.push_back(node -> val);
20             if(node -> left || node -> right) nextLevel = true;
21         }
22         else {
23             levelTraverse(node -> left, curLevel - 1, nextLevel, level);
24             levelTraverse(node -> right, curLevel - 1, nextLevel, level);
25         }
26     }