从m中选择n个元素

import java.util.ArrayList;import java.util.List;public class PickingATeam {public static void main(String[] args) {PickingATeam pick = new PickingATeam();List<String> ls = new ArrayList<String>();ls.add("A");ls.add("B");ls.add("C");ls.add("D");// ls.add("E");// ls.add("F");// ls.add("G");// ls.add("H");// ls.add("I");// ls.add("J");String[] s = ls.toArray(new String[4]);s = pick.pick(s, 0, ls.size(), 4);for (int i = 0; i < s.length; i++) {System.out.println(s[i]);}}public String[] pick(String[] ls, int start, int end, int teamNum) {// if the number you found is smaller than 0 ,it is invalid.// if the number you found is bigger than the length of collection.// invalidate!if (teamNum < 1 || teamNum > (end - start)) {return new String[0];}// if teamNum is 1, all the elements will be returned.if (teamNum == 1) {String[] candidate = new String[end - start];for (int i = 0; i < candidate.length; i++) {candidate[i] = ls[start + i];}return candidate;}// (n, k) = (n  1, k  1) + (n  1, k);// in the first part, only n-1 elements will be used. the n element will// combin will the (n  1, k  1)result.// this is that n element which is not included when picking the team.String FirstElem = ls[start];// getting (n-1,k-1);and combin with element n;String[] firPart = combin(FirstElem,pick(ls, start + 1, end, teamNum - 1));// getting (n  1, k);String[] secondPart = pick(ls, start + 1, end, teamNum);// code below are used to merge the result.int fristPartLength = firPart.length;int secondPartLength = secondPart.length;String[] pickingResult = new String[fristPartLength + secondPartLength];for (int i = 0; i < fristPartLength + secondPartLength; i++) {if (i < fristPartLength) {pickingResult[i] = firPart[i];} else {pickingResult[i] = secondPart[i - fristPartLength];}}return pickingResult;}/** * combin(a,[b,c,d]); return[ab,ac,ad] * */private String[] combin(String elem, String[] all) {String[] local = new String[all.length];for (int i = 0; i < all.length; i++) {local[i] = all[i];}for (int i = 0; i < local.length; i++) {local[i] = elem + local[i];}return local;}}