40. Combination Sum II

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

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

Note:

• All numbers (including target) will be positive integers.

• The solution set must not contain duplicate combinations.

Example 1:

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

Example 2:

Input: candidates = [2,5,2,1,2], target = 5,
A solution set is:
[
  [1,2,2],
  [5]
]

vector<vector<int>> combinationSum2(vector<int>& candidates, int target) { // time: O(2^n); space: O(2^n)
    sort(candidates.begin(), candidates.end());
    vector<vector<int> > res;
    vector<int> cur;
    helper(candidates, res, cur, 0, target);
    return res;
}
void helper(vector<int>& candidates, vector<vector<int> >& res, vector<int>& cur, int start, int target) {
    if (target == 0) {
        res.push_back(cur);
    } else {
        for (int i = start; i < candidates.size() && candidates[i] <= target; ++i) {
            if (i != start && candidates[i] == candidates[i - 1]) continue;
            cur.push_back(candidates[i]);
            helper(candidates, res, cur, i + 1, target - candidates[i]);
            cur.pop_back();
        }
    }
}