Recursive Functions in Tree Problems

Question

I have this fucntion that converts a Binary Tree to a Linked List. I am failing to understand how recursion works here since the flatten_helper function returns void. So what actually happens when a recursion function returns void? Can someone help me understand what happens in the code here?

Also, any links to understand recursion in tree problems would be highly appreciated as I am struggling to get a grasp on these types of problems.

void flatten_helper(struct TreeNode* root, struct TreeNode** prev)
{
    if (root == NULL) return;
    flatten_helper(root->right, prev);
    flatten_helper(root->left,  prev);    
    root->right = *prev;
    root->left = NULL;
    *prev = root;    
}
void flatten(struct TreeNode* root)
{
    struct TreeNode **prev; 
    prev = (struct TreeNode *)malloc(sizeof(struct TreeNode *));   
    *prev = NULL;
    flatten_helper(root, prev); 
    *prev = NULL;
    free(prev);
 }

Code Credits - Leetcode

One Guy Hacking · Accepted Answer · 2018-07-08 03:57:29Z

1

Don't think of flatten_helper() as a function, think of it as a procedure. It doesn't return a value, it actually does some work (it sets root->right to *prev, root->left to NULL, and *prev to root).

answered Jul 8, 2018 at 3:57

One Guy Hacking

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