创建 n 大小的排列

Question

I'm currently trying to make a method that will take a string and a number, and will then print all permutations of the string that are the size of the number. For example, permutation("barn", 3) would print "bar", "rab", "arn", and so forth. So far, I have this method which correctly print all permutations of a string:

public static void permutation(String prefix, String str) {
int n = str.length();
if (n == 0) System.out.println(prefix);
else {
    for (int i = 0; i < n; i++)
        permutation(prefix + str.charAt(i), str.substring(0, i) + str.substring(i+1, n));
}

Now I just need to implement the function that limits the size of each permutation, but I'm lost on what to do. Any help would be appreciated!

Answer 1

You just have to add a length parameter to the method, and print the prefix once it reaches that length:

public static void permutation(String prefix, String str, int len) {
    int n = str.length();
    if (prefix.length() == len) {
        System.out.println(prefix);
    } else {
        for (int i = 0; i < n; i++)
            permutation(prefix + str.charAt(i), str.substring(0, i) + str.substring(i+1, n), len);
    }
}

Now, if you execute

permutation("","barn",3);

you get:

bar
ban
bra
brn
bna
bnr
abr
abn
arb
arn
anb
anr
rba
rbn
rab
ran
rnb
rna
nba
nbr
nab
nar
nrb
nra