binaryTreeLevelOrderTraversal.swift

/*
 Given a binary tree, return the level order traversal of its nodes' values. (ie, from left to right, level by level).

 For example:
 Given binary tree [3,9,20,null,null,15,7],

 return its level order traversal as:

 [
 [3],
 [9,20],
 [15,7]
 ]

 https://leetcode.com/problems/binary-tree-level-order-traversal/
 */

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     public var val: Int
 *     public var left: TreeNode?
 *     public var right: TreeNode?
 *     public init(_ val: Int) {
 *         self.val = val
 *         self.left = nil
 *         self.right = nil
 *     }
 * }
 */
class Solution {
    func levelOrder(_ root: TreeNode?) -> [[Int]] {
        guard let root = root else { return [[Int]]() }
        
        var queue = [root]
        var result = [[Int]]()
        while !queue.isEmpty {
            let count = queue.count
            var level = [Int]()
            for i in 0..<count {            
                let node = queue.removeFirst()
                level.append(node.val)
                if let left = node.left {
                    queue.append(left)
                }
                if let right = node.right {
                    queue.append(right)
                }
            }
            result.append(level)
        }
        return result
    }
}