Algorithms—39.Combination Sum

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思路：动态规划。

public class Solution {    public List<List<Integer>> combinationSum(int[] candidates, int target) {    Arrays.sort(candidates);    return f(candidates, target, new HashMap<Integer, List<List<Integer>>>());    }    public List<List<Integer>> f(int[] candidates, int target,HashMap<Integer, List<List<Integer>>> map){    if (map.get(target)==null) {    List<List<Integer>> ans=new ArrayList<List<Integer>>();if (target<candidates[0]) {return ans;}for (int i = 0; i < candidates.length; i++) {int k=candidates[i];if (k<target) {List<List<Integer>> list=f(candidates,target-k,map);if (list!=null&&list.size()!=0) {for (int j = 0; j < list.size(); j++) {if (list.get(j).get(0)>=k) {List<Integer> l=new ArrayList<Integer>();l.add(k);l.addAll(list.get(j));ans.add(l);}}}}else if (k==target) {List<Integer> l=new ArrayList<Integer>();l.add(k);ans.add(l);}else {break;}}map.put(target, ans);return ans;}    return map.get(target);    }}

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