39. Combination Sum[Medium]

39. Combination Sum

Given an array of distinct integers candidates and a target integer target, return a list of all unique combinations of candidates where the chosen numbers sum to target. You may return the combinations in any order.

The same number may be chosen from candidates an unlimited number of times. Two combinations are unique if the
frequency
of at least one of the chosen numbers is different.

The test cases are generated such that the number of unique combinations that sum up to target is less than 150 combinations for the given input.

Constraints:

• 1 <= candidates.length <= 30
• 2 <= candidates[i] <= 40
• All elements of candidates are distinct.
• 1 <= target <= 40

Example

Input: candidates = [2,3,6,7], target = 7
Output: [[2,2,3],[7]]
Explanation:
2 and 3 are candidates, and 2 + 2 + 3 = 7. Note that 2 can be used multiple times.
7 is a candidate, and 7 = 7.
These are the only two combinations.

思路

题解

    public List<List<Integer>> combinationSum(int[] candidates, int target) {
        ArrayList<List<Integer>> res = new ArrayList<>();

        dfsCombinationSum(0, target, 0, candidates, new ArrayList<>(), res);

        return res;
    }

    public void dfsCombinationSum(int sum, int target, int index, int[] candidates, List<Integer> temp, List<List<Integer>> res) {
        if (sum == target) {
            res.add(new ArrayList<>(temp));
            return;
        }
        if (sum > target || index >= candidates.length)
            return;
        temp.add(candidates[index]);
        dfsCombinationSum(sum + candidates[index], target, index, candidates, temp, res);
        temp.remove(Integer.valueOf(candidates[index]));
//        temp.remove(temp.size() - 1);
	// 上面这个三步操作其实是一组，下面这一步是去下一阶段的trigger
        dfsCombinationSum(sum, target, index + 1, candidates, temp, res);
    }