LeetCode 168. Excel表列名称 Excel Sheet Column Title

【题目描述】
给定一个正整数，返回它在 Excel 表中相对应的列名称。
例如：

    1 -> A
    2 -> B
    3 -> C
    ...
    26 -> Z
    27 -> AA
    28 -> AB 
    ...

【示例1】

输入: 1
输出: "A"

【示例2】

输入: 28
输出: "AB"

【示例3】

输入: 701
输出: "ZY"

【思路】
1、其实就是求 10进制转26进制
2、注意：26进制没有26这个数字，就像2进制只有0 1没有2一个道理
3、时间复杂度O(log26n) 以26为底n的对数
4、空间复杂度O(1)

代码实现：

func convertToTitle(_ n: Int) -> String {
    if n == 0 { return "" }
    let arr = ["A","B","C","D","E","F","G","H","I","J","K","L","M","N","O","P","Q","R","S","T","U","V","W","X","Y","Z"]
    var res = String()
    var tmpN = n
    while tmpN > 0 {
        tmpN-=1//
        res = arr[tmpN%26] + res
        tmpN/=26
    }
    return res
}