Problem

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).

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]
]

Code

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
static int var=[](){
    std::ios::sync_with_stdio(false);
    cin.tie(NULL);
    return 0;
}();
class Solution {
public:
    vector<vector<int>> storage;
    vector<vector<int>> levelOrderBottom(TreeNode* root) {
        levelOrder(0,root);
        reverse(storage.begin(),storage.end());
        return storage;
    }
    void levelOrder(int level,TreeNode *node){
        if(node!=NULL){
            if(storage.size()<=level)
                storage.push_back(vector<int>());
            storage[level].push_back(node->val);
            levelOrder(level+1,node->left);
            levelOrder(level+1,node->right);
        }
    }
};

Result

107. Binary Tree Level Order Traversal II.png