LeetCode No145 Binary Tree Postorder Traversal

Given a binary tree, return the postorder traversal of its nodes' values.

For example:
Given binary tree {1,#,2,3},

   1    \     2    /   3

return [3,2,1].

Note: Recursive solution is trivial, could you do it iteratively?

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题目链接：https://leetcode.com/problems/binary-tree-postorder-traversal/

题目大意：求一个二叉树的后续遍历。

思路：递归。

附上代码：

/** * 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> postorderTraversal(TreeNode* root) {        vector <int> ans ;        if ( root == NULL )            return ans ;        postOrder ( root, ans ) ;        return ans ;    }    void postOrder ( TreeNode* root , vector <int>& ans )    {        if ( root == NULL )            return ;        postOrder ( root -> left , ans ) ;        postOrder ( root -> right , ans ) ;        ans.push_back ( root -> val ) ;    }};