maximumDepthOfBinaryTree.swift

/*
 Given a binary tree, find its maximum depth.

 The maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node.

 Note: A leaf is a node with no children.

 Example:

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

 return its depth = 3.

 https://leetcode.com/problems/maximum-depth-of-binary-tree/
 */

/**
 * 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 maxDepth(_ root: TreeNode?) -> Int {

        guard let root = root else { return 0 }

        return dfs(root, 1)
    }

    func dfs(_ node: TreeNode, _ level: Int) -> Int {

        if node.left == nil && node.right == nil { return level }

        var ll = level
        var lr = level

        if let left = node.left {
            ll = dfs(left, level+1)
        }

        if let right = node.right {
            lr = dfs(right, level+1)
        }

        return max(ll, lr)
    }
}