二叉树的最小深度

  3
   /   9  20
    /     15   7

func minDepth(_ root: TreeNode?) -> Int {
    guard let tree = root else { return 0 }
    if tree.left == nil, tree.right == nil {
        return 1
    }
    var minDeepCount = Int.max
    if let left = tree.left {
        minDeepCount = min(minDepth(left), minDeepCount)
    }
    if let right = tree.right {
        minDeepCount = min(minDepth(right), minDeepCount)
    }
    return minDeepCount + 1
}