Get missing letters - TopCoder challenge
Since the size of the alphabet is fixed, you don't need a set, a boolean array will be enough:


Initialize a boolean array, with size of the alphabet, all values false
For each letter in the input string, set the flag to true for index letter - 'A'
After the entire input string is processed, iterate over the boolean array, if the value is false for an index, then append the character index + 'A' to the list of missing letters


The time complexity of this alternative algorithm is \$O(n)\$ where \$n\$ is the size of the input string, space complexity is \$O(1)\$, for the size of the alphabet.