LeetCode OJ 144. Binary Tree Preorder Traversal

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     struct TreeNode *left;
 *     struct TreeNode *right;
 * };
 */
/**
 * Return an array of size *returnSize.
 * Note: The returned array must be malloced, assume caller calls free().
 */
int* preorderTraversal(struct TreeNode* root, int* returnSize) {
    int top = -1, i = 0;
    int *return_array = (int*)malloc(sizeof(int) * 1000);
    struct TreeNode *stack[1000];
    
    if(NULL == root){
        *returnSize = i;
        return return_array;
    }
    stack[++top] = root;
    while(-1 != top){
        root = stack[top--];
        return_array[i++] = root->val;
        if(NULL != root->right)
            stack[++top] = root->right;
        if(NULL != root->left)
            stack[++top] = root->left;
    }
    *returnSize = i;
    return return_array;
}