kotlin coding test course, lowest common ancestor

In this lecture, we will explore the “Lowest Common Ancestor” problem and explain step by step how to solve it using Kotlin.

Problem Description

It is a problem of finding the Lowest Common Ancestor (LCA) of two nodes A and B in a given binary tree. The LCA is the deepest node that includes both nodes simultaneously.

For example, let’s assume we have a binary tree like the one below:

        1
       / \
      2   3
     / \
    4   5
    

In this case, the LCA of nodes 4 and 5 is node 2. This is because node 2 is the parent of 4 and 5.

Input Format

The input consists of the root node of the binary tree and two nodes A and B. A and B are one of the nodes in the tree.

Output Format

The output is the LCA node of nodes A and B.

Problem Solving Strategy

1. Recursive Approach: We will explore the tree recursively using the characteristics of the binary tree.
2. Base Condition: If the current node is null or the current node is A or B, return that node.
3. Recursive Call: We will explore the left and right subtrees recursively.
4. Ancestor Determination: If the nodes returned from left and right are both not null, the current node is the LCA.

Code Implementation

Now let’s implement the above algorithm in Kotlin.

data class TreeNode(val value: Int, var left: TreeNode? = null, var right: TreeNode? = null)

fun lowestCommonAncestor(root: TreeNode?, A: TreeNode?, B: TreeNode?): TreeNode? {
    // Base Condition
    if (root == null || root == A || root == B) {
        return root
    }

    // Explore left and right subtrees
    val left = lowestCommonAncestor(root.left, A, B)
    val right = lowestCommonAncestor(root.right, A, B)

    // If current node is LCA
    return when {
        left != null && right != null -> root // Found in different subtrees
        left != null -> left // Found only in left subtree
        right != null -> right // Found only in right subtree
        else -> null // Not found
    }
}
    

Code Explanation

The code above can be divided into the following key parts:

• data class TreeNode: A data class defining a node in the binary tree.
• lowestCommonAncestor: A recursive function to find the lowest common ancestor.
• After checking the base condition, it explores the left and right subtrees and returns the LCA based on the conditions.

Time Complexity and Space Complexity

The time complexity of this algorithm is O(N). (N is the number of nodes) because it traverses all nodes.

The space complexity is O(H), where H corresponds to the height of the tree. This refers to the space used by the recursive call stack.

Example Input and Output

We can consider the following input:

    Input:
    Tree:
        1
       / \
      2   3
     / \
    4   5
    A = 4
    B = 5

    Output:
    2
    

Conclusion

In this lecture, we have looked in detail at the definition and solution methods for the Lowest Common Ancestor problem using Kotlin. This problem frequently arises in various situations, and by mastering binary tree traversal techniques, you will be equipped to solve many algorithmic problems.