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);
    }
    
};