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]
]

Difficulty:Easy

class Solution {
public:
vector<vector<int> > ans;
void push(TreeNode* root,int dep){
if(root==NULL)
return;
if(ans.size()>dep)
ans[dep].push_back(root->val);
else{
vector<int> v;
v.push_back(root->val);
ans.push_back(v);
}
push(root->left,dep+1);
push(root->right,dep+1);
}

vector<vector<int> > levelOrderBottom(TreeNode* root) {
push(root,0);
int len = ans.size();
vector<vector<int> > ans2;
for(int i = len-1;i>=0;i--)
ans2.push_back(ans[i]);
return ans2;
}

};