Solution.cs

public class Solution {
    private List<IList<int>> ans = new List<IList<int>>();
    private List<int> t = new List<int>();
    private int[] candidates;

    public IList<IList<int>> CombinationSum2(int[] candidates, int target) {
        Array.Sort(candidates);
        this.candidates = candidates;
        dfs(0, target);
        return ans;
    }

    private void dfs(int i, int s) {
        if (s == 0) {
            ans.Add(new List<int>(t));
            return;
        }
        if (i >= candidates.Length || s < candidates[i]) {
            return;
        }
        for (int j = i; j < candidates.Length; ++j) {
            if (j > i && candidates[j] == candidates[j - 1]) {
                continue;
            }
            t.Add(candidates[j]);
            dfs(j + 1, s - candidates[j]);
            t.RemoveAt(t.Count - 1);
        }
    }
}