在给定预序列的情况下，用C构造只有叶子的树

Question

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 1

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;
}