40. Combination Sum II Leetcode Java

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

Each number in C may only be used once in the combination.

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 10,1,2,7,6,1,5 and target 8,
A solution set is:
[1, 7]
[1, 2, 5]
[2, 6]
[1, 1, 6] 

Solution:

Classic recursion problem.

Almost the same as last problem Combination Sum

Difference: 
1. Element can only be used once and may have duplicated elements.
Tips:
1. The index for next level will be current index+1;
2. To avoid duplicated results: make sure if the iteration pointer i is not the start index, it will skip the duplicated element. Note: always execute the first loop(i==start).
eg. 2,2,2,3 target =7, the recursion tree will look like this:
2,2,2,3--> 2,2,3 --> 2,3, Notice: during the recursion, we skipped several 2,2,3 and 2,3

  public List<List<Integer>> combinationSum2(int[] candidates, int target) {  
    List<List<Integer>> res=new ArrayList<List<Integer>>();   
    if(candidates==null || candidates.length==0) return res;   
    Arrays.sort(candidates);   
    List<Integer> item=new ArrayList<Integer>();   
    helper(candidates,0,target,item,res);   
    return res;   
   }   
   public void helper(int[] candidates, int index, int target, List<Integer> item, List<List<Integer>> res){   
    if(target==0){   
     List<Integer> temp= new ArrayList<Integer>(item);   
     res.add(temp);   
     return;   
    }   
    for(int i=index;i<candidates.length;i++){   
     if(target-candidates[i]<0) return;   
     item.add(candidates[i]);   
     helper(candidates,i+1,target-candidates[i],item,res);   
     item.remove(item.size()-1);   
     while(i<candidates.length-1 && candidates[i]==candidates[i+1]) i++;  
    }   
   }