leetcode Binary Tree Postorder Traversal 二叉树后续遍历

先给出递归版本的实现方法，有时间再弄个循环版的。代码如下：

 1 /**
 2  * Definition for binary tree
 3  * struct TreeNode {
 4  *     int val;
 5  *     TreeNode *left;
 6  *     TreeNode *right;
 7  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 8  * };
 9  */
10 class Solution {
11 public:
12     vector<int> postorderTraversal(TreeNode *root) {
13         vector<int> nodeValues;
14         if (root == NULL) return nodeValues;
15         postorderTraversal_Rec(root,nodeValues);
16         
17         return nodeValues;
18     }
19 private:
20     
21     void postorderTraversal_Rec(TreeNode *root, vector<int>& nodeValues){
22         if (root == NULL) return;
23         if (root->left != NULL) postorderTraversal_Rec(root->left,nodeValues);
24         if (root->right != NULL)    postorderTraversal_Rec(root->right,nodeValues);
25         
26         nodeValues.push_back(root->val);
27     }
28     
29 };