0 I'm working on a leetcode problem: Given the root of a binary tree, return the inorder traversal of its nodes' values

Question

int *arr=NULL;
int size=0;
void inorder (struct TreeNode *root)
{
   if(root)
   {
       inorder(root->left);
       arr=(int *)realloc(arr,sizeof(int)*(++size));
       arr[size-1]=root->val;
       inorder(root->right);
   }
}
int* inorderTraversal (struct TreeNode* root, int* returnSize)
{
    inorder(root);
    *(returnSize)=size;
    return arr;
}

// I don't understand what's the mistake I have done, but it's giving me wrong answer. Test cases when run are passed successfully but during submission "Wrong Answer" Pops up. Can anybody help me solve this please?

Answer 1

You should not use global variables for this: their values will be retained from one test to the next, and so a subsequent test will start with the array values left by the previous run.

Instead, pass (the address of) the array and its size as arguments:

void inorder (struct TreeNode *root, int **arr, int *size)
{
   if (root)
   {
       inorder(root->left, arr, size);
       *arr = (int *)realloc(*arr, sizeof(int)*(++*size));
       (*arr)[*size - 1] = root->val;
       inorder(root->right, arr, size);
   }
}

int* inorderTraversal (struct TreeNode* root, int* returnSize)
{
    int *arr = NULL;
    *returnSize = 0;
    inorder(root, &arr, returnSize);
    return arr;
}