CareerCup

void reverse_string(char * string) 
{
	int n = strlen(string); // number of characters in the string
	for(int i = 0; i < n; ++i) {
		if((string[i] >= 'a') && (string[i] <= 'z')) {
			string[i] -= 32;
		} else if((string[i] >= 'A') && (string[i] <= 'Z')) {
			string[i] += 32;
		}
	}
	return;
}

public StringBuilder RetrunReverseStringCase(string value)
        {
            StringBuilder str = new StringBuilder();
            foreach (char c in value)
            {
                if (char.IsLetter(c))
                {
                    int numberEq = Convert.ToInt32(c);
                    if (c > 90)
                    {
                        numberEq = numberEq - 32;
                    }
                    else
                    {
                        numberEq = numberEq + 32;
                    }

                    str.Append(Convert.ToChar(numberEq));
                }
                else
                {
                    str.Append(c);
                }
            }
            return str;
        }

you don't need to use strlen.
you can use
while(*string!='\0')

string Reverse(string toReverse)
{
var sb = new StringBuilder();
for(int i = 0; i < toReverse.Length; i++)
{
if (toReverse[i].ToString() == toReverse[i].ToString().ToUpper())
{
sb.Append(toReverse[i].ToString().ToLower());
}
else
{
sb.Append(toReverse[i].ToString().ToUpper());
}
}
return sb.ToString();
}

unction revString(sample){
    var revSample="";
    var i=0;
    while(i<sample.length){
        if(sample.charAt(i)==(sample.charAt(i)).toUpperCase()){
           revSample= revSample.concat((sample.charAt(i)).toLowerCase());
        }else{
            revSample=revSample.concat((sample.charAt(i)).toUpperCase());
        }
     i++;   
    }
    return revSample;
}
var x=revString("Hello World");
alert(x);

In PHP:

<?php
function revString($sample){

$revSample = "";
$i=0;
while($i<strlen($sample)){
if($sample{$i} == strtoupper($sample{$i})){
$revSample = $revSample.(strtolower($sample{$i}));
}else{
$revSample=$revSample.(strtoupper($sample{$i}));
}
$i++;
}
return $revSample;
}
$x = revString("hELLo wOrLd");
echo $x;
?>

#include<stdio.h>
#include<conio.h>
#include<string.h>
void main()
{
char str[20];
int i,j,k,n;
clrscr();
printf("enter string\n");
gets(str);
for(i=0;str[i]!=0;i++)
{
if(str[i]>=65&&str[i]<=90)
str[i]+=32;
else if(str[i]>=97&&str[i]<=122)
str[i]-=32;
}
printf("new string\t");
puts(str);
getch();
}

public static void main (String[] args) throws java.lang.Exception
	{	
		String str = "hELLo wOrLd1";
		StringBuilder builder = new StringBuilder();
		char[] chars = str.toCharArray();

		for(int i = 0; i < chars.length; i++) {
			if(Character.isLetter(chars[i]) && Character.isLowerCase(chars[i])) {
				builder.append(String.valueOf(chars[i]).toUpperCase());
			} else {
				builder.append(String.valueOf(chars[i]).toLowerCase());
			}
		}
		
		System.out.println(builder.toString());
	}

public StringBuilder RetrunReverseStringCase(string value)
        {
            StringBuilder str = new StringBuilder();
            foreach (char c in value)
            {
                if (char.IsLetter(c))
                {
                    int numberEq = Convert.ToInt32(c);
                    if (c > 90)
                    {
                        numberEq = numberEq - 32;
                    }
                    else
                    {
                        numberEq = numberEq + 32;
                    }

                    str.Append(Convert.ToChar(numberEq));
                }
                else
                {
                    str.Append(c);
                }
            }
            return str;
        }

in JavaScript

function invertCase(text) {
  return text
           .split('')
           .map(char => char === char.toLowerCase() ? char.toUpperCase() : char.toLowerCase())
           .join('');
}