[LeetCode 第8题] -- Binary Tree Preorder Traversal

题目链接: Binary Tree Preorder Traversal

题目意思: 给定一个二叉树根节点，求前序序列

代码:

/**
 * 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 getPreOrder(TreeNode *root, vector<int> &preOrder);
    vector<int> preorderTraversal(TreeNode *root);
};

void Solution::getPreOrder(TreeNode *root, vector<int> &preOrder) {
     if (root != NULL) {
        preOrder.push_back(root->val);
        getPreOrder(root->left, preOrder);
        getPreOrder(root->right, preOrder);
     }
}

vector<int> Solution::preorderTraversal(TreeNode *root) {
     vector<int> preOrder;
     preOrder.clear();
     if (NULL == root) {
        return preOrder;
     }
     getPreOrder(root, preOrder);
     return preOrder;
}