I am solving Leetcode problem #94 for in-order traversal of a binary tree.

I cannot understand why - when I use &cpy (currently commented) when calling helper() the program works correctly but not when I use &gResult.

int countNode(struct TreeNode* root)
{
    if(root)
        return 1   countNode(root->left)   countNode(root->right);
    return 0;
}

void helper(struct TreeNode* root, int** res)
{
    if(root)
    {
        helper(root->left, res);
        *((*res)  ) = root->val;
        helper(root->right, res);
    }
}

/**
 * Note: The returned array must be malloced, assume caller calls free().
 */
int* inorderTraversal(struct TreeNode* root, int* returnSize){
    int *gResult = NULL; 
    *returnSize = countNode(root);
    gResult = (int*)malloc(((*returnSize)) * sizeof(int));
    memset(gResult, 0, (*returnSize) * sizeof(int));   
    int *cpy = gResult;
//    helper(root, &cpy);
    helper(root, &gResult);
    return gResult;
}

CodePudding user response：

The function helper changes the value of the pointer passed to the function by reference.

    *((*res)  ) = root->val;
      ^^^^^^^^

So after calling the function the original pointer will not point to the allocated memory.

Thus using an intermediate pointer like in this code snippet

int *cpy = gResult;
helper(root, &cpy);

leaves the pointer gResult unchanged that points to the dynamically allocated memory and is returned from the function inorderTraversal. And the caller of the function can be able to free the allocated memory successfully using the returned pointer.