leetcode-199-binary-tree-right-side-view.swift

/**

Copyright (c) 2020 David Seek, LLC

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https://leetcode.com/problems/binary-tree-right-side-view/


Given a binary tree, imagine yourself standing on the right side of it, return the values of the nodes you can see ordered from top to bottom.

Example:

Input: [1,2,3,null,5,null,4]
Output: [1, 3, 4]
Explanation:

   1            <---
 /   \
2     3         <---
 \     \
  5     4       <---

*/

/**
Big O Annotation
Time complexity O(n) where n is the amount of nodes in root.
Space complexity O(n) where n is the amount of nodes in nums.
*/
func rightSideView(_ root: TreeNode?) -> [Int] {

    // Without root nothing to see from the side
    guard let root = root else {
        return [] 
    }

    // Get the tree in level order traversal
    let levelTraversal = getLevelTraversal(of: root)

    // Get a result array of size levels
    var result: [Int] = Array(repeating: 0, count: levelTraversal.count)

    // Iterate through the traversal
    for (level, values) in levelTraversal {

        // And get the last element of each level
        result[level] = values.last ?? 0
    }

    return result
}

/**
Simple level order traversal:
Example: 

[
  0: [1]    
  1: [4,2]  
  2: [5,3]  
]
*/
func getLevelTraversal(of root: TreeNode) -> [Int: [Int]] {

    // The final output dictionary
    var levelTraversal: [Int: [Int]] = [:]

    // Init a queue
    var queue = Queue()

    // And enqueue the root at level 0
    queue.enqueue(root, at: 0)

    // While the queue is not empty
    while !queue.isEmpty {

        // Dequeue
        let (node, level) = queue.dequeue()!

        // Check if we already have a mapping for this level
        if let mapped = levelTraversal[level] {

            // If so, add nodes value
            levelTraversal[level] = mapped + [node.val]

        } else {

            // If not, set it
            levelTraversal[level] = [node.val]
        }

        // Enqueue the left child
        if let left = node.left {
            queue.enqueue(left, at: level + 1)
        }

        // And the right child
        if let right = node.right {
            queue.enqueue(right, at: level + 1)
        }
    }

    return levelTraversal
}

/**
Basic Queue Stack.
All operations are O(1) in time.
*/
struct Queue {

    private var enqueueStack: [(node: TreeNode, level: Int)] = []
    private var dequeueStack: [(node: TreeNode, level: Int)] = []

    init() {}

    public var isEmpty: Bool {
        return enqueueStack.isEmpty && dequeueStack.isEmpty
    }

    public mutating func enqueue(_ node: TreeNode, at level: Int) {
        enqueueStack.append((node: node, level: level))
    }

    public mutating func dequeue() -> (node: TreeNode, level: Int)? {

        if dequeueStack.isEmpty {

            dequeueStack = enqueueStack.reversed()
            enqueueStack.removeAll()
        }

        return dequeueStack.popLast()
    }
}