二叉树的前序遍历（力扣第144题）

/**
 * 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 preorder(struct TreeNode* proot,int* parr,int* size)
 {
     if(proot==NULL)
        return;
    parr[(*size)++]=proot->val;
    preorder(proot->left,parr,size);
    preorder(proot->right,parr,size);
 }
int* preorderTraversal(struct TreeNode* root, int* returnSize) {
   
    int* arr=(int*)malloc(sizeof(int)*2000);
    * returnSize=0;
    preorder(root,arr,returnSize);
    return arr;
}