题目要求：

   1
    \
     2
    /
   3

思路：

递归遍历。前，中，后序只是顺序改变了一下，前序是a.push_back在两个递归之前，中序是在中间，后序是在之后。

代码：

/**
 * 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: A Tree
     * @return: Preorder in ArrayList which contains node values.
     */
     
    vector<int> a;
    void recursion(TreeNode * root){
        if(root == NULL){
            return;
        }
        a.push_back(root->val);
        recursion(root->left);
        recursion(root->right);
    }
    vector<int> preorderTraversal(TreeNode * root) {
        // write your code here
        recursion(root);
        return a;
    }
};