Question

While printing Binary Search Tree(BST) using recursive function (pre-order). I need to print all the parents(path form root) of current node.
An auxiliary data structure can(e.g. path in my code) be use but I don’t want to keep node->path to store path.

      4                           
     / \  
    /   \  
   2     6
  / \   / \
 1   3  5  7  

Suppose I am printing nodes in rows using pre-order traverse:

NODE    PATH  
4       4  
2       4,2  
1       4,2,1
3       4,2,3       
6       4,6
5       4,6,5
7       4,6,7  

I did as follows: Working fine!
Path end with 0 (Zero) value in this code. And there is no node value is 0 in BST.

void printpath(int* mypath){
   while(*mypath)  
      printf("%d ", *mypath++);  
}

void preorder(struct tree *p, int* path){
    int *mypath = calloc(sizeof(path)/sizeof(int) + 1 , sizeof(int*));
    int* myp=mypath;

    if(p!=NULL){  
       while( *myp++ = *path++ );  
       --myp;
       *myp=p->data;
       *(myp+1)=0;

        printf("%d PATH ",p->data);
        printpath(mypath);
        printf("\n");
        preorder(p->left, mypath);
        preorder(p->right, mypath);
    }
    free(mypath);
}

But I don’t want to keep path array as there is lots of nodes in BST. Can some one suggest me other data-structure/ or method ? A suggestion would be enough but should be efficient.

Answer 1

Here’s an old trick, which still works: keep the back pointers in the call stack.

    struct stacked_list{
      struct stacked_list* prev;
      struct tree* tree; 
    };

   void printpath_helper(int data, struct stacked_list* path) {
      if (!path->prev)
        printf("%d PATH ", data);
      else
        printpath_helper(data, path->prev);
      printf("%d ", path->tree->data);
    }

    void printpath(struct stacked_list* path) {
      printpath_helper(path->tree->data, path);
      putchar('\n');
    }

    void preorder_helper(struct stacked_list* path) {
      if (path->tree) {
        printpath(path);
        struct stacked_list child = {path, path->tree->left};
        preorder_helper(&child);
        child.tree = path->tree->right;
        preorder_helper(&child);
      }
    }

    void preorder(struct tree* tree) {
      struct stacked_list root = {NULL, tree};
      preorder_helper(&root);
    }

Each recursion of preorder_helper creates an argument struct and passes its address to the next recursion, effectively creating a linked list of arguments which printpath_helper can walk up to actually print the path. Since you want to print the path from top to bottom, printpath_helper needs to also reverse the linked list, so you end up doubling the recursion depth of the function; if you could get away with printing bottom to top, printpath_helper could be a simple loop (or tail recursion).