CombinationToSum.java

package test;import java.util.ArrayList;import java.util.List;import java.util.Stack;/** * http://bylijinnan.iteye.com/blog/1347441 *  * @author ZengWenFeng * */public class CombinationToSum{/* * 第21 题 2010 年中兴面试题 编程求解： 输入两个整数 n 和 m ，从数列 1 ， 2 ， 3.......n 中随意取几个数 , 使其和等于 m , * 要求将其中所有的可能组合列出来 . two solutions permutation01:Recursion.easy to write and read-->pick n or * not,haha permutation02:put n,then put n-1...if bigger,remove;if smaller,keep putting;if * ok,output. */public static void main(String[] args){CombinationToSum cts = new CombinationToSum();//permutation01(int sum,int n)  cts.permutation01(10, 10);System.out.println("===========");cts.permutation02(10, 6);}/* * Recursion.use Stack<Integer> we can use ArrayList,too. private List<Integer> list=new * ArrayList<Integer>(); list.add(n); list.remove(list.indexOf(n)); */private Stack<Integer> stack = new Stack<Integer>();public void permutation01(int sum, int n){if (n <= 0 || sum <= 0)return;if (sum == n){printStack(stack);System.out.print(n);System.out.println();}stack.add(n);permutation01(sum - n, n - 1);stack.pop();permutation01(sum, n - 1);}public void permutation02(int sum, int n){if (n <= 0 || sum <= 0)return;for (int i = n; i > 0; i--){if (i == sum){System.out.println(i);continue;}List<Integer> list = new ArrayList<Integer>();list.add(i);for (int j = i - 1; j > 0;){list.add(j);int ret = isOK(list, sum);if (ret < 0){j--;}if (ret == 0){printList(list);System.out.println();j = list.get(1) - 1;//now we go back and make the second element smaller  list.clear();list.add(i);}if (ret > 0){list.remove(list.size() - 1);//too large,remove the last element  j--;}}}}// whether the sum of list element equals to sum or not  public static int isOK(List<Integer> list, int sum){int re = 0;int total = 0;for (int each : list){total += each;}if (total > sum)re = 1;if (total < sum)re = -1;return re;}public void printStack(Stack<Integer> stack){/* * while(!stack.isEmpty()){ int temp=stack.pop(); System.out.print(temp+" "); } *///don't remove the elements in stack  for (Integer each : stack){System.out.print(each + " ");}}public void printList(List<Integer> list){for (int each : list){System.out.print(each + " ");}}}

109 18 27 37 2 16 46 3 15 4 15 3 24 3 2 1===========6 4 6 3 1 5 4 1 5 3 2 4 3 2 1