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.

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

Notice

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

Example

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

Solution: Backtracking.

public List<List<Integer>> combinationSum(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>();
    dfs(candidates, target, res, item, 0);
    return res;
}
public void dfs(int[] candidates, int target, List<List<Integer>> res, List<Integer> item, int index) {
    if(target == 0) {
        res.add(new ArrayList<>(item));
        return ;
    }
    if(target < 0) {
        return ;
    }
    for(int i = index; i < candidates.length; i++) {
        if(i > index && candidates[i] == candidates[i - 1]) {
            continue;
        }
        item.add(candidates[i]);
        dfs(candidates, target - candidates[i], res, item, i);
        item.remove(item.size() - 1);
    }
}