[LeetCode]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) {        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);    }};