LeetCode -- Combination Sum II
2015-10-12 11:01:28         来源：Stay focus,Stay Crazy

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.

Note:
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.
For example, given candidate set 10,1,2,7,6,1,5 and target 8,
A solution set is:
[1, 7]
[1, 2, 5]
[2, 6]
[1, 1, 6]

1.本题解法依旧是回溯，使用一个数组arr在遍历nums时，添加nums[i]，进入递归，移除nums[i]。i∈[0,n)
2.使用index变量来track下次循环的起始位置。
3.使用哈希来去重

```    public IList> CombinationSum2(int[] candidates, int target)
{
if(candidates == null || candidates.Length == 0){
return null;
}

var arr = candidates.OrderBy(x=>x).ToList();

var result = new Dictionary>();
Travel(arr ,new List(), 0, target, result);

return new List>(result.Select(x=>x.Value));
}

private void Travel(IList candidates, IList arr, int index, int target, Dictionary> result){
if(target == 0 ){
var key = string.Join(,, arr);
if(!result.ContainsKey(key)){
}

return ;
}

for(var i = index ;i < candidates.Count; i++){
if(target < candidates[i]){
return;
}

Travel(candidates, arr, i + 1, target - candidates[i], result);
arr.Remove(candidates[i]);
}
}```