LeetCode 39 - Combination Sum

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] 

Solution:

public List<List<Integer>> combinationSum(int[] candidates, int target) {
    List<List<Integer>> result = new ArrayList<>();
    Arrays.sort(candidates);
    combine(result, new ArrayList<Integer>(), candidates, target, 0);
    return result;
}

private void combine(List<List<Integer>> result, List<Integer> list, int[] num, int target, int pos) {
    if(target < 0) return;
    if(target == 0) {
        result.add(new ArrayList<Integer>(list));
        return;
    }
    for(int i=pos; i<num.length; i++) {
        list.add(num[i]);
        combine(result, list, num, target-num[i], i); //注意，从i开始，不是i+1，因为可以重复使用
        list.remove(list.size()-1);
    }
}