[144] Binary Tree Preorder Traversal

意外的順利（？）真是嚇到我了。

該驚嚇的是，這題妳竟然還沒有寫過吧XD

/**

* Definition for a binary tree node.

* struct TreeNode {

* int val;

* struct TreeNode *left;

* struct TreeNode *right;

* };

*/

/**

* Note: The returned array must be malloced, assume caller calls free().

*/

void dfs(struct TreeNode* root, int *arr, int *idx)

{

if (root == NULL)

return;

arr[*idx] = root->val;

(*idx)++;

dfs(root->left, arr, idx);

dfs(root->right, arr, idx);

return;

}

int* preorderTraversal(struct TreeNode* root, int* returnSize){

int *ret = calloc (101, sizeof(int));

int idx =0;

dfs(root, ret, &idx);

*returnSize = idx;

return ret;

}