171. Excel Sheet Column Number

problem

solution

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

analysis

• time complexity O(n)
• space complexity O(1)