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：

 /**
  * Definition for binary tree
  * struct TreeNode {
  *     int val;
  *     TreeNode *left;
  *     TreeNode *right;
  *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
  * };
  */
 class Solution {
 public:
     vector<int> postorderTraversal(TreeNode *root) {
         vector<int> nodeValues;
         if (root == NULL) return nodeValues;
         postorderTraversal_Rec(root,nodeValues);

         return nodeValues;
     }
 private:

     void postorderTraversal_Rec(TreeNode *root, vector<int>& nodeValues){
         if (root == NULL) return;
         if (root->left != NULL) postorderTraversal_Rec(root->left,nodeValues);
         if (root->right != NULL)    postorderTraversal_Rec(root->right,nodeValues);

         nodeValues.push_back(root->val);
     }

 };