[LeetCode]61. Excel Sheet Column Number Excel列序号

Related to question Excel Sheet Column Title

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 

Credits:
Special thanks to @ts for adding this problem and creating all test cases.

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解法：相当于26进制转10进制。

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

不使用pow函数的改进：

class Solution {
public:
    int titleToNumber(string s) {
        int n = s.size(), res = 0, i = 0;
        for (; i < n - 1; ++i) { //注意不能循环到n-1，防止溢出
            res += (toupper(s[i]) - 'A' + 1);
            res *= 26;
        }
        return res + (toupper(s[i]) - 'A' + 1);
    }
};