/********************************
 * Example of a fragile program
 * for ECS 153. Find the flaws!
 * 
 * Till Stegers, 10 Jan 2006
 * 
 * http://wwwcsif.cs.ucdavis.edu/~stegers/ecs153/fragile.c
 * 
 ********************************/

#include <stdio.h>
#include <string.h>
#include <stdlib.h>

/* Data structure for tree nodes */
typedef struct {
    char label;
    struct NODE* left;
    struct NODE* right;
} NODE;


/* Construct a binary tree from prefix notation.
 * Leafs are denoted as nodes with empty subtrees,
 * as in (a()()) for the singleton tree containing a.
 * example: (b(a()())(c(d()())(e()()))) corresponds to
 * 
 *        b
 *    a       c
 *          d   e
 *
 * empty tree: ()
 * singleton tree: (a()())
 */
NODE* parse_tree(char** prefix){
    NODE *node;
 
    /* empty tree */
    if ((*prefix)[1] == ')'){
        /* printf("empty tree: ()\n"); */
        *prefix += 2;
        return NULL;
    }

    node = (NODE*) malloc(sizeof(NODE));
    
    /* get root */
    node->label = (*prefix)[1];
    /* printf("Node: %c\n", node->label); */

    /* skip '(' and node label */
    *prefix += 2;
    
    node->left =  (struct NODE*) parse_tree(prefix);
    node->right = (struct NODE*) parse_tree(prefix);
    
    /* account for closing paren */
    *prefix += 1;
    
    return node;
    
}

/* Free memory occupied by tree */
void free_tree(NODE* root){
    if (root == NULL) {
        printf("Error! Tried to free NULL!\n");
        return;
    }

    printf("(%c", root->label);

    if (root->left != NULL)
        free_tree((NODE*)root->left);
    else
        printf("()");
        
    if (root->right != NULL)
        free_tree((NODE*)root->right);
    else
       printf("()");
    
    free(root);
    
    printf(")");
}


/* 
 * Read in prefix traversal, construct tree,
 * free tree, then exit.
 */
int main(int argc, char **argv) {
    
    char input[100];
    NODE* root;
    char * prefix;
    
    printf("Please enter tree in prefix traversal: \n");
    while (gets(input) == NULL){
        printf("Please enter tree in prefix traversal: \n");
    }
    
    prefix = (char*) malloc(strlen(input));
    strcpy(prefix, input);
    root = parse_tree(&prefix);
    
    if (root != NULL){
        printf("Successfully created tree\n");
    
        /* work with tree ... */
 
        /* free tree */
        printf("Freeing tree...\n");
        free_tree((NODE*)root);
        printf("\n");
     }
     else {
        printf("Tree empty or in wrong format\n");
        return -1;
     }

    return 0;
}