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.
• 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]
]

解题思路为backtracking。

public class Solution {
    public List<List<Integer>> combinationSum(int[] candidates, int target) {
        List<List<Integer>> res = new ArrayList<List<Integer>>();
        Arrays.sort(candidates);
        helper(candidates, target, 0, new ArrayList<Integer>(), res);
        return res;
    }
    
    private void helper(int[] candidates, int target, int index, List<Integer> sofar, List<List<Integer>> res) {
        if(target < 0) return;
        if(index >= candidates.length) return;
        if(target == 0) {
            res.add(new ArrayList<Integer>(sofar));
            return;
        }
        // with current candidate
        sofar.add(candidates[index]);
        helper(candidates, target-candidates[index], index, sofar, res);
        sofar.remove(sofar.size()-1);
        // without current candidate
        helper(candidates, target, index+1, sofar, res);
    }
}