# Leetcode problem - Vertical Order Traversal of a Binary Tree

I am getting a following error on leetcode :-

Line 1034: Char 9: runtime error: reference binding to null pointer of type ‘std::vector<int, std::allocator>’ (stl_vector.h) SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior /usr/bin/…/lib/gcc/x86_64-linux-gnu/9/…/…/…/…/include/c++/9/bits/stl_vector.h:1043:9

Kindly Suggest a possible correction to my code

My Code :-

``````      /**
* Definition for a binary tree node.
* struct TreeNode {
*     int val;
*     TreeNode *left;
*     TreeNode *right;
*     TreeNode() : val(0), left(nullptr), right(nullptr) {}
*     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
*     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left),
right(right) {}
* };
*/
class Solution {
public:
vector<vector<int>> verticalTraversal(TreeNode* root)
{

vector<vector<int>> ans;

map<int, vector<int>> mp;

if(root == NULL)
return ans;

queue<pair<TreeNode* , int>> q;
q.push({root,0});

while(!q.empty())
{
auto p= q.front();
TreeNode *curr = p.first;
int hd=  p.second;
q.pop();

if(root->left != NULL)
{
q.push({root->left, hd-1});
}

if(root -> right != NULL)
{
q.push({root->right, hd+1});
}
ans[hd].push_back(curr->val);
}

return ans;
}
};
``````

Try initialising the vector or handle the case where index hd is not there in ans.

``````if (hd >= ans.size())
{
vector<int> temp;
temp.push_back(curr->val);
//if hd == y and vector.size == x, insert an empty vector at indices (x,y]
ans.push_back(temp);
}
else
{
ans[hd].push_back(curr->val);
}``````