168. Excel Sheet Column Title

Given a positive integer, return its corresponding column title as appear in an Excel sheet.

For example:

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

Example 1:

Input: 1
Output: "A"

Example 2:

Input: 28
Output: "AB"

Example 3:

Input: 701
Output: "ZY"

来自 <https://leetcode.com/problems/excel-sheet-column-title/description/>

思路1：这相当于一个26进制的转换问题，但特殊之处在于，它是从A开始到Z结束，按照题目规定A为1,Z为26，而不是一般情况下的从0到25。我们仍然可以先进行普通的26进制转换，之后再处理这一问题。

 1 class Solution(object):
 2     def convertToTitle(self, n):
 3         """
 4         :type n: int
 5         :rtype: str
 6         """
 7         yushu = []
 8         while n > 0:
 9             yushu.append(n % 26)
10             n = n // 26
11         for i in range(len(yushu)-1):
12             if yushu[i] <= 0:
13                 yushu[i+1] = yushu[i + 1] - 1
14                 yushu[i] = yushu[i] + 26
15         yushu=yushu[::-1]
16         if yushu[0] == 0:
17             del (yushu[0])
18         s = ''
19         for i in yushu:
20             s += chr(i + 64)
21         return s

思路2：上面的解法在逻辑上有点模糊，其实对于每一次循环n应该等于n-1因为，这样就能将1到26转换为0到25

 1 class Solution(object):
 2     def convertToTitle(self, n):
 3         """
 4         :type n: int
 5         :rtype: str
 6         """
 7         yushu=[]
 8         while n>0:
 9             yushu.append((n-1)%26+65)
10             n = (n-1)//26
11         yushu=yushu[::-1]
12         result=''
13         for i in yushu:
14             result+=chr(i)
15         return result

思路3：对于这种重复执行问题，可以用递归方法处理

 1 class Solution(object):
 2     def convertToTitle(self, n):
 3         """
 4         :type n: int
 5         :rtype: str
 6         """
 7         if n == 0:
 8             return ""
 9         else:
10             return self.convertToTitle((n - 1) // 26)+chr((n - 1) % 26 + ord('A'))