OJ lintcode 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。

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/**
 * 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.
     */
     void preorder_tra(TreeNode * p,vector & vi){
        if(p!=NULL){
            vi.push_back(p->val);
            preorder_tra(p->left,vi);
            preorder_tra(p->right,vi);
        
        }
        
    }
     
    vector preorderTraversal(TreeNode *root) {
        // write your code here
        vector vi;
        preorder_tra(root,vi);
        return vi;
    }
};