Binary Tree Preorder Traversal

二叉树的前序遍历
递归实现

/**

 * Definition for binary tree

 * struct TreeNode {

 *     int val;

 *     TreeNode *left;

 *     TreeNode *right;

 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}

 * };

 */

class Solution

{

public:

    void PreOrder(TreeNode* root,vector<int>&vec)

    {

        if(root!=NULL)

        {

            vec.push_back(root->val);

            PreOrder(root->left,vec);

            PreOrder(root->right,vec);

        }

    }

    vector<int> preorderTraversal(TreeNode *root)

    {

         vector<int>vec;

         PreOrder(root,vec);

         return vec;

    }

};