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