171. Excel Sheet Column Number

171. Excel Sheet Column Number

class Solution {
public:
    int titleToNumber(string columnTitle) {
        int len = columnTitle.size();
        int sum = columnTitle.back() - 'A' + 1;
        int tmp = 1;
        for(int i = len - 2; i >= 0; --i)
            sum += (columnTitle[i] - 'A' + 1) * (tmp *= 26);
        return sum;
    }
};

26进制转为10进制，就n转10差不多

但我还是算的更麻烦了

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

review

class Solution {
public:
    int titleToNumber(string columnTitle) {
        int ret{ 0 };
        for(auto ch : columnTitle)
            ret = ret * 26 + (ch - 'A' + 1); // 注意这个括号是必要的，因为加法符号是左结合，所以如果没括号会先加ch导致溢出。
        return ret;
    }
};