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66. 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。
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;
* }
* }
*/
class Solution {
public:
/**
* @param root: A Tree
* @return: Preorder in ArrayList which contains node values.
*/
void pre(TreeNode *head,vector<int> &a){
a.push_back(head->val);
if(head->left!=NULL)pre(head->left,a);
if(head->right!=NULL)pre(head->right,a);
}
vector<int> preorderTraversal(TreeNode * root) {
// write your code here
vector<int> v;
if(root==NULL)return v;
pre(root,v);
return v;
}
};