171. excel table column No

171. Excel Table column No

Original title link ：https://leetcode.cn/problems/excel-sheet-column-number/

Give you a string columnTitle , Express Excel Column names in the table . return The column sequence number corresponding to the column name .

for example ：

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

Example 1:

Input : columnTitle = “A”
Output : 1
Example 2:

Input : columnTitle = “AB”
Output : 28
Example 3:

Input : columnTitle = “ZY”
Output : 701

Tips ：

1 <= columnTitle.length <= 7
columnTitle Composed of capitalized English only
columnTitle In scope [“A”, “FXSHRXW”] Inside

Their thinking ：

This question is to 26 Hexadecimal letters are converted into corresponding decimal numbers , First convert the letters to the corresponding 26 Base integers , Then multiply these integers by the corresponding base number from low to high , convert to 10 Hexadecimal number , Pay attention here columnTitle The rightmost letter is the lowest order , A flashback traversal is required .

Code implementation ：

class Solution:
    def titleToNumber(self, columnTitle: str) -> int:
        ans = 0
        # 26 Base of base 
        mul = 1

        #  Flashback traversal columnTitle, Because the smallest number is on the far right 
        for i in range(len(columnTitle) - 1, -1, -1):
            #  To convert to decimal numbers , To calculate ASCII Value 
            cur = ord(columnTitle[i]) - ord('A') + 1
            ans += cur * mul
            mul *= 26
        
        return ans

reference ：
https://leetcode.cn/problems/excel-sheet-column-number/solution/excelbiao-lie-xu-hao-by-leetcode-solutio-r29l/