Solution:

Cosider A,B,C.... Z as 1,2,3.... 26

AB => A * 26^1 + B * 26^0 => 26 + 2 => 28

AAC => A * 26^2 + A * 26^1 + C * 26^ 0 => 705

"class Solution {

public:

int titleToNumber(string s) {

int N = s.size();

if(!N) return 0;

int val = 1;

int result = 0;

for(int i=N-1;i>=0;i--)

{

result = result + ((s[i]-65+1)*val);

val = val * 26;

}

return result;

}

};"