Problem

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.

 Notice

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]
]

思路

和 Subsets II 差不多

    public List<List<Integer>> combinationSum2(int[] nums, int target) {
        List<List<Integer>> result = new ArrayList<>();
        if (nums == null || nums.length == 0) {
            return result;
        }

        Arrays.sort(nums);

        helper(nums, result, new ArrayList<Integer>(), target, 0);
        return result;
    }

    private void helper(int[] nums, List<List<Integer>> result, List<Integer> path, int target, int startIndex) {

        int index = startIndex;
        while (index < nums.length) {
            path.add(nums[index]);
            target -= nums[index];

            if (target == 0) result.add(new ArrayList<Integer>(path));
            if (target > 0) helper(nums, result, path, target, index + 1);

            target += nums[index];
            path.remove(path.size() - 1);

            index++;
            while (index < nums.length && nums[index] == nums[index - 1]) index++;
        }
    }