lintcode.66 二叉树前序遍历

/**
 * 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> l;
    vector<int> preorderTraversal(TreeNode * root) {
        // write your code here
        if(root==NULL)
            return l;
        l.push_back(root->val);
        preorderTraversal(root->left);
        preorderTraversal(root->right);
        return l;
    }
};