Print all possible combinations in an array (which may have duplicate elements) of N.

Input:

{1, 2, 3, 4}, r = 2

Output:

{1, 2}, {1, 3}, {1, 4}, {2, 3}, {2, 4} and {3, 4 }.

Package recursion; import Java. util. arraylist; import Java. util. collections; public class outputs {/* input: {1, 2, 3, 4}, r = 2 output: {1, 2}, {1, 3}, {1, 4}, {2, 3}, {2, 4} and {3, 4 }. */public static void main (string [] ARGs) {int [] S1 = {1, 2, 3, 4 }; arraylist <integer> set1 = new arraylist <integer> (); For (int I: S1) {set1.add (I);} int R1 = 2; arraylist <integer> ret1 = new arraylist <integer> (); rec2 (set1, R1, 0, ret1, new arraylist <integer> ()); system. out. println (ret1); int [] S2 = {1, 2, 1, 3, 2}; arraylist <integer> set2 = new arraylist <integer> (); for (int I: S2) {set2.add (I);} int r2 = 2; arraylist <integer> ret2 = new arraylist <integer> (); collections. sort (set2); rec2 (set2, R2, 0, ret2, new arraylist <integer> (); system. out. println (ret2);} // solution 1: analysis: the output is available. The first element added to Al is selected cyclically, and then the subinterval is reduced, recursive processing of subproblems // first is the first element currently processing subintervals public static void Rec (arraylist <integer> set, int R, int first, arraylist <integer> ret, arraylist <integer> Al) {If (Al. size () = r) {// exit condition: Exit RET when the number in Al reaches R. add (New arraylist <integer> (Al); Return ;}for (INT I = first; I <set. size (); I ++) {// from the first element of the subinterval to the last element will have the opportunity to be added to the Al if (I + 1 <set. size () & set. get (I + 1) = set. get (I) {// Skip duplicate elements, but check whether the row is out of bounds;} Al. add (set. get (I); // note that the parameters used in the for loop are I rather than firstrec (set, R, I + 1, RET, Al ); // shorten the subinterval by 1 distance to Al. remove (Al. size ()-1) ;}}// solution 2: select or do not select an element public static void rec2 (arraylist <integer> set, int R, int first, arraylist <integer> ret, arraylist <integer> Al) {If (r = Al. size () {// exit condition: when the number of ALS reaches R, RET can be exited. add (New arraylist <integer> (Al); return;} If (first> = set. size () {// return if the first traversal exceeds the set capacity;} while (first + 1 <set. size () & set. get (first + 1) = set. get (first) {// skip repeated elements, but check whether the cross-border first ++;} rec2 (set, R, first + 1, RET, Al ); // do not select the current element Al. add (set. get (first); // select the current element rec2 (set, R, first + 1, RET, Al); Al. remove (Al. size ()-1 );}}

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