#68 Binary Tree Postorder Traversal

題目描述：

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

Have you met this question in a real interview? 

Yes

Example

Given binary tree {1,#,2,3},

   1
    \
     2
    /
   3

 

return [3,2,1].

題目思路：

這題就是傳統的tree遍歷，用了傳統的recursion。

Mycode（AC = 10ms）：

/**
 * Definition of TreeNode:
 * class TreeNode {
 * public:
 *     int val;
 *     TreeNode *left, *right;
 *     TreeNode(int val) {
 *         this->val = val;
 *         this->left = this->right = NULL;
 *     }
 * }
 */
class Solution {
    /**
     * @param root: The root of binary tree.
     * @return: Postorder in vector which contains node values.
     */
public:
    vector<int> postorderTraversal(TreeNode *root) {
        // write your code here
        vector<int> ans;
        if (root == NULL) return ans;
        
        postorderTraversal(ans, root);
        return ans;
    }
    
    void postorderTraversal(vector<int>& ans, TreeNode *root) {
        if (root == NULL) {
            return;
        }
        
        // order: left->right->root
        postorderTraversal(ans, root->left);
        postorderTraversal(ans, root->right);
        ans.push_back(root->val);
    }
};