C语言

#define NULL ((void *)0)

// 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().
 */
int* preorderTraversal(struct TreeNode* root, int* returnSize){
    int* res = (int*)malloc(sizeof(int)*101);
    *returnSize=0;
    preorder(root, res, returnSize);
    return res;
}

void preorder(struct TreeNode* root, int* res, int* resSize){
    if(!root)
        return;
    res[(*resSize)++]=root->val;
    preorder(root->left, res, resSize);
    preorder(root->right, res, resSize);
}