Find out all combinations and permutations - Java

问题

I am all new here and also a beginner. I am working on a personal project (developing an app for personal use). I am a bit stuck and yes, I did search around but couldn't find much. My aim is to find all possible combinations. Let's say I have a finite number of lists defined like:

List<String> AList contains {"a1","a2","a3","a4"} 
List<String> BList contains {"b1","b2","b3"} 
List<String> CList contains {"c1","c2","c3","c4","c5"} 
List<String> DList contains {"d1","d2"}

I would like to find all combinations with:

1) the combination a1 a2 vein the same as a2 a1 and so on

2) the number of elements per result set not being fixed: a combination possibility is per example merging all lists into one.. or each element in a list of its own or merging two elements in a list and the rest in another.. and so on..

I know it has to be a recursive function.. but that how far i am right now.. Any help would be appreciated. Thanks

回答1:

i did a similar thing for myself, this however would give you permutations of a1 a2 etc and not combinations, meaning the order does not matter.

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

}

With this logic, you would have to turn your Lists into one long string and then send this method your string

回答2:

Here is a utility class I implemented for this purpose a while ago. It uses array of int instead of list of String but it should give you the idea:

public final class Math {

    private Math() {
    }

    public static int[][] ncks(int n, int k) {
        int[] ks = new int[k];
        for (int i = 0; i < ks.length; i++) {
            ks[i] = i + 1;
        }

        int[][] ncks = new int[nck(n, k)][];

        boolean done = false;
        for (int j = 0; !done; j++) {
            ncks[j] = Arrays.copyOf(ks, ks.length);
            done = getNext(ks, n, k);
        }
        return ncks;
    }

    private static int nck(int n, int k) {
        int d = 1;
        int r = 1;

        int m = java.lang.Math.max(k, n - k) + 1;
        for (; m <= n; m++, d++) {
            r *= m;
            r /= d;
        }
        return r;
    }

    private static boolean getNext(int[] num, int n, int k) {
        int target = k - 1;

        num[target]++;
        if (num[target] > ((n - (k - target)) + 1)) {
            while (num[target] > ((n - (k - target)))) {
                target--;
                if (target < 0) {
                    break;
                }
            }

            if (target < 0) {
                return true;
            }

            num[target]++;
            for (int i = target + 1; i < num.length; i++) {
                num[i] = num[i - 1] + 1;
            }
        }
        return false;
    }

}

来源：https://stackoverflow.com/questions/17134155/find-out-all-combinations-and-permutations-java

标签

java

combinations

permutation