Sort letters in alphabetical order

Sort the letters of a given string in alpabetical order.

Sample Code:

function sortLetter(str) {
// Enter your logic here...
}

Examples:

Input: 'codetisan'
Output: 'acdeinost'
Explanation: The letters of the output are sorted in alphabetical order.

Input: 'letter'
Output: 'eelrtt'
Explanation: The letters of the output are sorted in alphabetical order.