171. Excel Sheet Column Number

问题

Related to question Excel Sheet Column Title
Given a column title as appear in an Excel sheet, return its corresponding column number.

例子

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

分析

字符映射，逐字符遍历字符串，然后转成1-26即可。之前的结果在下一轮循环中乘26.

要点

字符映射

时间复杂度

O(n)

空间复杂度

O(1)

代码

class Solution {
public:
    int titleToNumber(string s) {
        int res = 0, n = s.size();
        for (int i = 0; i < n; i++) {
            res = res * 26 + s[i] - 'A' + 1;
        }
        return res;
    }
};