171. Excel Sheet Column Number - Easy

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

For example:

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

Example 1:

Input: "A"
Output: 1

Example 2:

Input: "AB"
Output: 28

Example 3:

Input: "ZY"
Output: 701

 

Constraints:

• 1 <= s.length <= 7
• s consists only of uppercase English letters.
• s is between "A" and "FXSHRXW".

 

convert base-10 number system to base-26 number system

time = O(n), space = O(1)

class Solution {
    public int titleToNumber(String s) {
        int res = 0, base = 1;
        for(int i = s.length() - 1; i >= 0; i--) {
            res += base * (s.charAt(i) - 'A' + 1);
            base *= 26;
        }
        return res;
    }
}