171. Excel Sheet Column Number

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

int titleToNumber(string s) {
   int res = 0;
    for(int i = 0; i < s.length(); i++){
        int d = s[i] - 'A' + 1;
        res = res * 26 + d;
    }
    return res;
}

更优化的代码

int titleToNumber(string s) {
    int res = 0;
    for(int i = 0; i < s.length(); i++){
        res = res * 26 - 'A' + s[i] + 1;
    }
    return res;
}