How to convert a binary tree to binary search tree in-place, i.e., we cannot use any extra space

Question

How to convert a binary tree to binary search tree in-place, i.e., we cannot use any extra space.

Answer 1

#include 
#include 

typedef int data_t;

struct tree_node {
    struct tree_node * left;
    struct tree_node * right;
    data_t data;
};

        /* a bonsai-tree for testing */
struct tree_node nodes[10] =
{{ nodes+1, nodes+2, 1}
,{ nodes+3, nodes+4, 2}
,{ nodes+5, nodes+6, 3}
,{ nodes+7, nodes+8, 4}
,{ nodes+9, NULL, 5}
,{ NULL, NULL, 6}
,{ NULL, NULL, 7}
,{ NULL, NULL, 8}
,{ NULL, NULL, 9}
        };

struct tree_node * harvest(struct tree_node **hnd)
{
struct tree_node *ret;

while (ret = *hnd) {
        if (!ret->left && !ret->right) {
                *hnd = NULL;
                return ret;
                }
        if (!ret->left ) {
                *hnd = ret->right;
                ret->right = NULL;;
                return ret;
                }
        if (!ret->right) {
                *hnd = ret->left;
                ret->left = NULL;;
                return ret;
                }
        hnd = (rand() &1) ? &ret->left : &ret->right;
        }

return NULL;
}

void insert(struct tree_node **hnd, struct tree_node *this)
{
struct tree_node *ret;

while ((ret= *hnd)) {
        hnd = (this->data  < ret->data ) ? &ret->left : &ret->right;
        }
*hnd = this;
}

void show(struct tree_node *ptr, int indent)
{
if (!ptr) { printf("Null\n"); return; }

printf("Node(%d):\n", ptr->data);
printf("%*c=", indent, 'L');  show (ptr->left, indent+2);
printf("%*c=", indent, 'R');  show (ptr->right, indent+2);
}

int main(void)
{
struct tree_node *root, *this, *new=NULL;

for (root = &nodes[0]; this = harvest (&root);  ) {
        insert (&new, this);
        }

show (new, 0);
return 0;
}