Combination Sum --- LeetCode

Given a set of candidate numbers (C) and a target number (T), find all unique combinations in C where the candidate numbers sums to T.

The same repeated number may be chosen from C unlimited number of times.

Note:

• All numbers (including target) will be positive integers.
• Elements in a combination (a1, a2, … , ak) must be in non-descending order. (ie, a1 ≤ a2 ≤ … ≤ ak).
• The solution set must not contain duplicate combinations.

For example, given candidate set 2,3,6,7 and target 7, 
A solution set is: 
[7] 
[2, 2, 3] 

解题思路：使用回溯法，深度优先搜索。

public class Solution {
    
 public List<List<Integer>> combinationSum(int[] candidates, int target) {              
           Set<List<Integer>> result=new HashSet<List<Integer>>();
           int len=candidates.length;
           if(len==0||candidates==null){
           return null;
           }
           Arrays.sort(candidates);
           Stack<Integer> stack=new Stack<Integer>();
           count(candidates,0,target,stack,result);
           
      return new ArrayList<List<Integer>>(result);
      
}      
 
public void count(int [] candidates,int start, int target,Stack<Integer> stack,Set<List<Integer>> result){
 if(target==0){
@SuppressWarnings("unchecked")
List<Integer> tmp=(List<Integer>)stack.clone();  
result.add(tmp);
 }   
 for(int i=start;i<candidates.length;i++){
 stack.push(candidates[i]);
 if(target-candidates[i]>=0){  
 count(candidates,i,target-candidates[i],stack,result);
 }
 stack.pop();
 }


}
    
    
}