145. Binary Tree Postorder Traversal

想不懂为什么是hard。。。

/**
 * 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<int>ve;

    void posto(TreeNode* root){
        if(root == NULL) return ;
        posto(root -> left);
        posto(root -> right);
        ve.push_back(root -> val);
    }

    vector<int> postorderTraversal(TreeNode* root) {
        posto(root);

        return ve;
    }
};