[LeetCode#171]Excel Sheet Column Number

This problem is very easy. If you truely understand the principle of binary or decimal system.
To represent a number a in the base of b. we have
a0 a1 a2.....
Covert it into decimal system, we have
(b^0)*a0 + (b^1)*a1 + (b^2)*a2 + ....

In this problem, base is 26 (‘A‘ to ‘Z‘).
For the digit at index ‘i‘, it is weight is:
int digit_weight = (int)Math.pow(26, s.length() - 1 - i);

One thing should be very careful, in this mapping.
‘A‘ is mapping to ‘1‘ rather than ‘0‘, which means it starts from ‘1‘ to ‘26‘. 
The decimal number is just a way to represent it! Just as the mapping we have done before (Hash). 
The important thing is to be able to recover it from the decimal number. 


Note:
In java, we don‘t have power method fo integer, we have to use force coverion here.
(int)Math.pow(26, s.length() - 1 - i);

static double pow(double a, double b)
Returns the value of the first argument raised to the power of the second argument.

public class Solution {
    public int titleToNumber(String s) {
        if (s == null)
            throw new IllegalArgumentException("the passed in argument is invalid.");
        int res = 0;
        for (int i = 0; i < s.length(); i++) {
            int digit_weight = (int)Math.pow(26, s.length() - 1 - i);
            res += (s.charAt(i) - ‘A‘ + 1) * digit_weight;
        }
        return res;
    }
}