Permutations of capitalization

Question

I want to build a list containing every possible permutation of capitalization of a word. so it would be

List permutate(string word)
{
    List         


        

Answer 1

Assuming:

1) You don't care too much that this is O(n*2^n)... although I'm curious to know: what is the best asymptotic run time for this type of problem?

2) Your input is all lowercase.

3) Your input is < 32 characters long. (# of usable bits in the permutation counter, i)

    List permutate(string word)
{
    List ret = new List();

// MAGIC HAPPENS HERE
Dictionary toUppers = new Dictionary(26);
toUppers.Add('a', 'A');
toUppers.Add('b', 'B');
toUppers.Add('c', 'C');
toUppers.Add('d', 'D');
toUppers.Add('e', 'E');
toUppers.Add('f', 'F');
toUppers.Add('g', 'G');
toUppers.Add('h', 'H');
toUppers.Add('i', 'I');
toUppers.Add('j', 'J');
toUppers.Add('k', 'K');
toUppers.Add('l', 'L');
toUppers.Add('m', 'M');
toUppers.Add('n', 'N');
toUppers.Add('o', 'O');
toUppers.Add('p', 'P');
toUppers.Add('q', 'Q');
toUppers.Add('r', 'R');
toUppers.Add('s', 'S');
toUppers.Add('t', 'T');
toUppers.Add('u', 'U');
toUppers.Add('v', 'V');
toUppers.Add('w', 'W');
toUppers.Add('x', 'X');
toUppers.Add('y', 'Y');
toUppers.Add('z', 'Z');

char[] wordChars = word.ToCharArray();
int len = wordChars.Length;

// iterate the number of permutations
for(int i = 0; i < 2^len; i++) {
    char[] newWord = new char[len]();

    // apply "i" as a bitmask to each original char
    for(int n = 0; n < newWord.Length; n++) {
        if((1 << n) & i != 0) {
            newWord[n] = toUppers[wordChars[n]]; // or skip the dictionary and just call Char.ToUpper(wordChars[n])
        } else {
            newWord[n] = wordChars[n];
        }
    }
    ret.Add(new String(newWord));
}

    return ret;
}

Note: I haven't compiled or tested this code. This is also implementing the bitwise compare that sharptooth suggested above.