leetcode: Binary Tree Postorder Traversal

后序，按照左子结点->右子结点->根节点的顺序即可

/**
 * 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> res;
        if( root == NULL )
        {
            return res;
        }
        res = fun( root , res );
        return res;
    }
    vector<int> fun( TreeNode *root,vector<int>res )
    {
        if( root == NULL )
        {
            return res;
        }
        if( root->left != NULL )
        {
            //res.push_back( root->left->val );
            res = fun( root->left,res );
        }
        if( root->right != NULL )
        {
            //res.push_back( root->right->val );
            res = fun( root->right,res );
        }
        res.push_back(root->val);
        return res;
    }
};