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) {
int d = depth(root);
vector<vector<int>> ret(d);
levelOrder(ret, root, d-1);
return ret;
}
int depth(TreeNode *root) {
if (!root) return 0;
return max(depth(root->left),depth(root->right))+1;
}
void levelOrder(vector<vector<int>> &ans, TreeNode *node, int level) {
if (!node) return;
ans[level].push_back(node->val);
levelOrder(ans,node->left,level-1);
levelOrder(ans,node->right,level-1);
}
};