Leetcode 107. Binary Tree Level Order Traversal II

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> > retVal;
levelOrder(root, retVal, 0);
reverse(retVal.begin(), retVal.end());
return retVal;
}

void levelOrder(TreeNode* root, vector<vector<int> > &v, int currLevel) {
if (root == NULL)
return;
if (v.empty() || currLevel > (v.size() - 1))
v.push_back(vector<int>());
v[currLevel].push_back(root->val);
levelOrder(root->left, v, currLevel + 1);
levelOrder(root->right, v, currLevel + 1);
}
};