9147 分段错误：11 从中序和前序遍历构造二叉树

Question

Given inorder and preorder traversal of a Binary Tree, construct the Binary Tree.

While constructing a tree from inorder and preorder traversal, i think i have written it properly but why is it giving a segmentatation fault. Please help. below is my code for the same:

#include<bits/stdc++.h>
using namespace std;

class Node {
 public :
 int data;
 Node* left;
 Node* right;

 Node(int data)
 {
    this->data = data;
    left = right = NULL;
 }
};

void Inorder(Node* root)
{
  if (root == NULL)
  {
    return;
  }

  Inorder(root->left);
  cout << root->data << " ";
  Inorder(root->right);
}

 Node* buildTreeHelper( int inorder[], int preorder[], int InS, int InE, int PreS, int PreE)
 {

  if (InS > InE)
  {
     return NULL;
  }

 int rootData = preorder[PreS];

 int lIns = InS;
 int rootIndex = -1;
 for (int i = InS ; i < InE ; i++)
  {
     if (inorder[i] == rootData)
       {
         rootIndex = i;
         break;
       }
  }
    int lIne = rootIndex - 1;
    int lPres = PreS + 1;
    int lPree = lIne - lIns + lPres;
    int rIns = rootIndex + 1;
    int rIne = InE;
    int rPres = lPree  + 1;
    int rPree = PreE;


   Node* root = new Node(rootData);
   root->left = buildTreeHelper(inorder, preorder, lIns, lIne, lPres, lPree);
   root->right = buildTreeHelper(inorder, preorder, rIns, rIne, rPres, rPree);

    return root;
}

Node* buildTree(int inorder[], int preorder[], int size)
{
    return buildTreeHelper(inorder, preorder, 0, size - 1, 0, size - 1);
}
int main()
{

    int inorder[] = {4, 2, 5, 1, 8, 6, 9, 3, 7};
    int preorder[] = {1, 2, 4, 5, 3, 6, 8, 9, 7};

   Node* root = buildTree(inorder, preorder, 9);
    Inorder(root);
    delete root;
    return 0;
}

I am unable to identify what's the error, please help me out of this.

Answer 1

It seems to me that at least one of your problems is that in the loop to find the root index in the in-order list of data, you don't actually check the last element (ie it should be i <= InE .)

Also, I can't say that your lPree calc is correct, because the names you have chosen are so unhelpful that I don't even want to read that line.

And, please don't use #include <bits/stdc++.h> . I don't know which compiler/IDE is it that encourages use of this particular header, but I've seen it in other (beginner) people's code and in my opinion, it's no good. Please just use standard headers. In this case, that would be iostream (maybe with a using std::cout; thrown in.)

By the way, at the end of the program, you are only freeing the single root node. That is obviously insufficient and incorrect. You have to traverse and free the whole tree (you could do that from the Node destructor, but I wouldn't recommend a recursive call in a destructor. You might be OK in this simple program.)