16. Flatten BST to sorted list

The problem can be found at the following link: Question Link

My Approach

To flatten a BST into a sorted list, we can perform an in-order traversal of the BST. During the traversal, we keep track of the previous and next nodes to adjust the pointers accordingly. The steps are as follows:

• Initialize head and tail pointers as NULL.

• Perform in-order traversal of the BST.

• While traversing, adjust the pointers of the nodes to form a linked list.

• Finally, return the head of the linked list.

Time and Auxiliary Space Complexity

• Time Complexity: The time complexity of the in-order traversal is O(n), where n is the number of nodes in the BST.

• Auxiliary Space Complexity: The space complexity is O(h), where h is the height of the BST.

Code (C++)

class Solution {
public:
    Node* head;
    Node* tail;
    void inOrder(Node* curr){
        if(!curr)
            return;
        
        inOrder(curr->left);
        if(!head)
            head = curr;
        else{
            tail->right = curr;
            curr->left = tail;
        }
        tail = curr;
        inOrder(curr->right);
    }
    Node *flattenBST(Node *root)
    {
        head = tail = NULL;
        inOrder(root);
        return head;
    }
};

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