0102-binary-tree-level-order-traversal.swift

class Solution {
    func levelOrder(_ root: TreeNode?) -> [[Int]] {
        guard let root = root else { return [] }
        var queue = [TreeNode]()
        var result = [[Int]]()
        queue.append(root)
        while !queue.isEmpty {
            var levelCount = queue.count
            var levelNodes = [Int]()
            while levelCount > 0 {
                let node = queue.first!
                queue.removeFirst()
                levelCount -= 1
                levelNodes.append(node.val)
                if let left = node.left {
                    queue.append(left)
                }

                if let right = node.right {
                    queue.append(right)
                }
            }
            result.append(levelNodes)
        }
        return result
    }

}