leetcode_num168&171_excel title&number

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

class Solution {
public:
    string convertToTitle(int n) {
        string s;
        while(n){
            s.insert(s.begin(),(n-1)%26+'A');//在字符串起始位插入
            n=(n-1)/26;
        }
        return s;
    }
};

由于没有0，所以需要先减1

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

class Solution {
public:
    int titleToNumber(string s) {
        int rs=0,j,a;
        for(int i=0;i<s.length();i++){
            j=s.length()-i-1;
            a=s[j]-'A'+1;
            rs+=(a*pow(26,i));
        }
        return rs;
    }
};