Complete Linked-List-2

[paridhimalviya]

Problem1 (https://leetcode.com/problems/binary-search-tree-iterator/)

Problem2 (https://leetcode.com/problems/reorder-list/)

Problem3 (https://practice.geeksforgeeks.org/problems/delete-without-head-pointer/1)

Problem4 (https://leetcode.com/problems/intersection-of-two-linked-lists/)

[super30admin]

Strengths:

• The student understands the concept of using a stack to simulate in-order traversal.
• The dfs function is implemented correctly in terms of the algorithm (pushing left nodes).

Areas for improvement:

1. Use Swift's built-in array as a stack: Instead of implementing a custom stack with a linked list, use an array. This is more efficient and simpler. For example:

   var stack: [TreeNode] = []

   Then use stack.append(node) for push and stack.removeLast() for pop.

2. Avoid force-unwrapping: Always safely unwrap optionals. In the next() method, check if the popped value is nil before using it.

3. Focus on the problem: The solution should only include code relevant to the BSTIterator. Remove unrelated code from other problems.

4. Correct the dfs implementation: The current dfs function is almost correct, but it should push the node before moving to the left. However, with the custom stack, it might not work as expected. Using an array-based stack will simplify this.

5. Follow Swift naming conventions: Function and variable names should be in camelCase. The dfs function is appropriately named, but the stack variable should be named clearly (e.g., stack).

6. Implement the stack without a linked list: The custom linked list stack is overkill. Here's how you can implement it with an array:

Revised solution sketch:

class BSTIterator {
    private var stack: [TreeNode] = []

    init(_ root: TreeNode?) {
        self.dfs(root)
    }
    
    func next() -> Int {
        let node = stack.removeLast()
        dfs(node.right)
        return node.val
    }
    
    func hasNext() -> Bool {
        return !stack.isEmpty
    }
    
    private func dfs(_ root: TreeNode?) {
        var current = root
        while let node = current {
            stack.append(node)
            current = node.left
        }
    }
}

[super30admin]

Strengths:

• You have understood the core idea of using a stack to simulate in-order traversal.
• The hasNext() method is correctly implemented to check if the stack is empty.
• You attempted to write a generic stack class, which shows good programming practice.

Areas for Improvement:

1. Stack Implementation: Instead of implementing a custom stack with a linked list, you should use Swift's built-in array with append for push and popLast for pop. This is more efficient and less error-prone. For example:

   var stack: [TreeNode] = []

   Then you can push with stack.append(node) and pop with stack.popLast().

2. Handling Optionals: Avoid force-unwrapping optionals unless you are absolutely sure they are not nil. In your next() method, you force-unwrap the popped node and its value, which could lead to crashes if the stack contains a nil node. Instead, you should safely unwrap the optional.

3. DFS Function: Your dfs function should be modified to work with non-optional nodes. The while loop condition while let node = curr is correct, but inside the loop, you should push the node without wrapping it in an optional since your stack should store non-optional nodes. Also, ensure that you are pushing the current node and then moving to the left child.

4. Code Simplicity: The problem does not require a generic stack. You can directly use an array of tree nodes. This simplifies the code and reduces the chance of errors.

5. Correctness: The next() method should first pop the node, then process the right subtree by calling dfs on the right child. Your current implementation does this, but the stack implementation might be pushing nil values, which causes issues.

6. Follow-up Requirements: The reference solution meets the follow-up requirements of O(1) amortized time for next() and hasNext() and O(h) space. Your solution, when corrected, can also meet these.

Suggested corrected code:

class BSTIterator {
    private var stack: [TreeNode] = []
    
    init(_ root: TreeNode?) {
        dfs(root)
    }
    
    func next() -> Int {
        let node = stack.removeLast()
        dfs(node.right)
        return node.val
    }
    
    func hasNext() -> Bool {
        return !stack.isEmpty
    }
    
    private func dfs(_ root: TreeNode?) {
        var current = root
        while let node = current {
            stack.append(node)
            current = node.left
        }
    }
}

Note: This assumes that the TreeNode class is defined as in the problem. Also, in Swift, using removeLast on an array is efficient for stack operations.