#66 Binary Tree Preorder Traversal

題目描述：

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

Have you met this question in a real interview? 

Yes

Example

Given:

    1
   / \
  2   3
 / \
4   5

return [1,2,4,5,3].

題目思路：

還是和#68一樣，調一下順序。

Mycode（AC = 15ms）：

/**
 * Definition of TreeNode:
 * class TreeNode {
 * public:
 *     int val;
 *     TreeNode *left, *right;
 *     TreeNode(int val) {
 *         this->val = val;
 *         this->left = this->right = NULL;
 *     }
 * }
 */

class Solution {
public:
    /**
     * @param root: The root of binary tree.
     * @return: Preorder in vector which contains node values.
     */
    vector<int> preorderTraversal(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: root->left->right
        ans.push_back(root->val);
        postorderTraversal(ans, root->left);
        postorderTraversal(ans, root->right);
        
    }
};