Getting Character as a numeric value

Sometimes when we need to get the numeric character into a numeric value, we might be using

char ch = '9';
int num = Character.getNumericValue(ch);

But if we are working on something like stack-based problems where we convert the String 3[a20[c]] to charArray , then the 20 will be divided into two characters like 2|0. The value of 20 is missing and the .getNumericValue(ch) does not work here.

String s = "20";
char[] ch = ['2','0'];
int num = 0;

while (ch.isEmpty()) {
    num = num * 10 + ch - '0';
}

1st iteration --> (0 * 10) + (2 - '0') = 0 + 2 = 2

2nd iteration --> (2 * 10) + (0 - '0') = 20 + 0 = 20

What does ch-'0' even mean?

It is doing ASCII calculations. Every character has an ASCII value associated with it,0 character ASCII value is 48 and 2 character ASCII value is 50. So when we perform the operation ch-'0', what it is doing is 50-48, i.e 2.

This particular trick is used in the following questions:

Decode String - LeetCodeLeetCode