lintcode 二叉树前序遍历

二叉树的前序遍历 

 

给出一棵二叉树，返回其节点值的前序遍历。

您在真实的面试中是否遇到过这个题？ 

Yes

样例

给出一棵二叉树 {1,#,2,3},

   1
    \
     2
    /
   3

 返回 [1,2,3].

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

//递归
class Solution {
public:
    /*
     * @param root: A Tree
     * @return: Preorder in ArrayList which contains node values.
     */
     
     void inorder (TreeNode *root, vector<int> &result) {
         result.push_back(root->val);
         if (root->left != NULL) {
             inorder(root->left, result);
         }
         
         if (root->right != NULL) {
             inorder(root->right, result);
         }
     }
     
    vector<int> preorderTraversal(TreeNode * root) {
        // write your code here
        vector<int> result;
        if (root == NULL) {
            return result;
        }
        inorder(root, result);
        return result;
    }
};