2^N-2的N次方

string powint(int n)//类似于字符串乘法{string result = "1";for (int i = 0; i < n; i++){string tmp="";int sum = 0, add = 0;for (int j = result.size() - 1; j >= 0; j--){sum=(result[j] - '0') * 2 + add;tmp = char(sum % 10+'0')+tmp;         //字符串拼接；add = sum / 10;}if (add>0)//最后一次是否有进位{tmp = char(add + '0') + tmp;}result = tmp;}return result;}