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we need to parse the string in the reverse order and iterate from all the characters and subtract 48 from each character and build the required number.

???

sysout(Integer.parseInt(str);

I dont know if you meant we should also avoid the Math and the String library functions -

String x = "121";
		int res= 0;
		for(int i=0;i<x.length();++i){
			int y = (int)(x.charAt(i)-'0');
			res= (res*10)+y;
		}

int conv(char *a)
{int num=0;
int ten=1;
for(int i=strlen(a)-1;i>=0;i--,ten*=10)//go from last digit
{
num+=(a[i]-'0')*ten;
}
return num;
}

<pre lang="java" line="1" title="CodeMonkey13178" class="run-this">import java.util.*;
import java.lang.*;

class Main
{
public static void main (String[] args) throws java.lang.Exception
{
System.out.print("Hello World");
}
}
</pre><pre title="CodeMonkey13178" input="yes">
</pre>

As an interviewer, I would be very happy if the candidate would ask stuff like:
- can I be sure the input is valid?
- should I handle negative numbers as well?

These are not complicated changes to the algorithm, and they show 'wide' thinking of the candidate

How do we handle negative numbers?

it is atoi() function implementation.

int y = (int)(x.charAt(i)-'0');
Can some one tell me what is actually going on here??
Why to do char-'0'

(int)x.charAt(i) gives the ascii value of the character.

so if you have 1 at charAt(0), the output is 49. ascii value of 0 is 48. So if we do ascii of 1 - ascii of 0, then we are doing 49 - 48 and the end result is saved in int y which will be 1. Applies same on any number.