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,null,null,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:
    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);
}

vector<vector<int>> levelOrderBottom(TreeNode* root) {
    int d = depth(root);
    vector<vector<int>> ans(d,vector<int> {});
    levelOrder(ans,root,d-1);
    return ans;
}
};