Extract Characters at Prime Indices

Given a string 'STR' of length 'N', return a new string containing all characters that are located at prime indices in the original string. Ensure that the order of characters in the new string is the same as their order in the original string.

Example:

Input:

STR = "abcdefg"

Output:

"bdf"

Explanation:

Characters at prime indices 2, 3, and 5 are 'b', 'd', and 'f', respectively.

Constraints:

• The first character of the string is at index 0 (0-based indexing).
• Prime indices are those indices that are prime numbers. For example, if a character is at index 5, it is at a prime index because 5 is a prime number.
• The string contains characters from 'a'-'z', 'A'-'Z', and '0'-'9'.