Keep on iterating the given string:

If the corresponding character is uppercase then convert it to lowercase using built in functions

Else if the character is lowercase then convert it to uppercase using built in functions.

If any whitespace then keep it as it is.

Return the final string

Code

#include <bits/stdc++.h>
using namespace std;
class Solution{
void solve(string str, int n) {
  for (int i = 0; i < n; i++) {
    int ascii = (int) str[i];  // Get ASCII value of character

    if (ascii >= 65 && ascii <= 90)  // if uppercase
      str[i] = towlower(str[i]);     // convert to lowercase

    else if (ascii >= 97 && ascii <= 122)  // if lowercase
      str[i] = towupper(str[i]);          // convert to uppercase
  }

  cout << "Resultant string: " << "\n";
  cout << str << "\n";
}

int main() {
  // Input string
  string str = "take u forward IS Awesome";
  int n = str.length();

  solve(str, n);
  return 0;
}
public class Solution{
    static void solve(String str, int n) {
        StringBuilder result = new StringBuilder();

        for (int i = 0; i < n; i++) {
            char ch = str.charAt(i);       // Get character
            int ascii = (int) ch;          // Get ASCII value

            if (ascii >= 65 && ascii <= 90)  // if uppercase
                result.append(Character.toLowerCase(ch)); // convert to lowercase

            else if (ascii >= 97 && ascii <= 122)  // if lowercase
                result.append(Character.toUpperCase(ch)); // convert to uppercase

            else
                result.append(ch);  // leave other characters 