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;    }};