How would you implement a StringToInt from scratch?

sYou are given a integer in string format;
String sNum = "25";
int iNum = StrToInt(sNum);

How does StrToInt() work?
Crudely it's as follows.
You go digit by digit. Each digit has a hex value.

0   0x30   48
1   0x31   49
2   0x32   50
3   0x33   51
4   0x34   52
....
9   0x39   57

The algorithm pseduo-code is as follows.

1. result = 0;
2. for each character i in sNum
3. result = result*10;
4.   result = result + ASCIIVal(sNum[i]) - ASCIIVal('0');

Let's take 25 for example.
For the first iteration;
sNum[0] = 2;
result = 0*10=0;
result = 0 + (50-48) = 2;

For the second iteration;
sNum[1] = 5;
result=2*10=20;
result=20+(53-48)=25

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