Combination Sum II

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

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

• All numbers (including target) will be positive integers.
• Elements in a combination (a1, a2, … , ak) must be in non-descending order. (ie, a1 ≤ a2 ≤ … ≤ ak).
• The solution set must not contain duplicate combinations.

Example:

Given candidate set [10,1,6,7,2,1,5] and target 8,

A solution set is:

[
  [1,7],
  [1,2,5],
  [2,6],
  [1,1,6]
]

Solution:

This question is similar to k Sum II. However, we need to avoid duplicate combinations. Therefore, we should try each unique number at every level of dfs.

Code:

public class Solution {

    public List<List<Integer>> combinationSum2(int[] num, int target) {
        List<List<Integer>> res = new ArrayList<>();
        Arrays.sort(num);
        dfs(num, target, 0, new ArrayList<Integer>(), res);
        return res;
    }

    private void dfs(int[] num, int target, int index, List<Integer> path, 
                        List<List<Integer>> res) {
        if (target == 0) {
            res.add(new ArrayList<>(path));
            return;
        }
        if (index >= num.length || target < 0) {
            return;
        }
        int prev = -1;
        for (int i = index; i < num.length; i++) {
            if (num[i] != prev) {
                path.add(num[i]);
                dfs(num, target - num[i], i + 1, path, res);
                prev = num[i];
                path.remove(path.size() - 1);
            }
        }
    }
}