Jeff Greene Jeff Greene - 29 days ago 9
C Question

Constructing a tree in C with only leaves, given a pre order sequence

I am trying to build a specific type of tree, where each internal node's data relies on the leaves of the tree. I have a pre-order array,

pre[] = { 0,0,0,1,2,3,0,4,5}
where each "0" represents an internal node, and anything else represents leaves. If I were to construct this tree, it would look like this:

0
/ \
0 0
/ \ / \
0 3 4 5
/ \
1 2


I am having trouble with the recursive call, specifically in the while loop. After I finish with creating the node with data "2", after I return the root I am left at the internal node right above it. This internal node will continue to run in the while loop and mess up my tree. If I decide to place a return root within the while loop at the end of the condition statements then the construction will stop after filling up the left side of the tree. How can I fix this?

1 #include <stdio.h>
2 #include <stdlib.h>
3 #include <limits.h>
4
5 struct node
6 {
7 int data;
8 int leafNode;
9 struct node *left;
10 struct node *right;
11 };
12
14 struct node* newNode (int data){
15 struct node* temp = (struct node *) malloc(sizeof(struct node));
16 temp->data =data;
17 temp->left = temp->right= NULL;
18 return temp;
19 }
22 struct node* _constructTree( int pre[], int * preIndex, int low, int high, int size, int leaf){
23 if (*preIndex >= size || low > high){
24 return NULL;}
25
26 printf("We are about to create this node: %d\n", pre[*preIndex]);
27
28 struct node* root = newNode(pre[*preIndex]);
29 *preIndex = *preIndex + 1;
30 if(leaf){
31 printf("Returning this leaf node: %d\n", pre[*preIndex - 1]);
32 return root;}
33
34
35 if(low == high){
36 return root;}
37
38 while(low<high){
39 if(pre[*preIndex] == 0){
40 root->left = _constructTree(pre, preIndex, *preIndex, high, size, 0);}
41
42 if(root->left == NULL){
43 root->left = _constructTree(pre, preIndex, *preIndex, high, size, 1);
44 return root;
45 }
46 if(pre[*preIndex] != 0){
47 root->right = _constructTree(pre, preIndex, *preIndex, high, size, 1);
48 return root;
49 }
50 else{
51 root->right = _constructTree(pre,preIndex, *preIndex, high, size, 0);}
52 }
53 return root;
54 }
55 }
56
57 struct node * constructTree(int pre[], int size){
58 int preIndex = 0;
59 return _constructTree(pre, &preIndex, 0, size -1, size, 0);
60 }
61

Answer

Would something like this not work (slightly summarized but hopefully it shows the idea)?:

construct_tree(int pre[], int* index){
    root=pre[*index];
    *index++;
    if(root>0)return root;
    left=construct_tree(pre,index);
    right=construct_tree(pre,index);
    return root;
}