public IList <IList <int> > FourSum(int[] nums, int target)
{
//Array.Sort(nums);
var hashSet = new HashSet <NumSet>();
var result = new List <IList <int> >();
for (int i = 0; i < nums.Length; i++)
{
var threeSum = ThreeSum(nums, target - nums[i], i);
if (threeSum.Count > 0)
{
foreach (var threeruplet in threeSum)
{
threeruplet.Add(nums[i]);
var set = new NumSet(threeruplet);
if (!hashSet.Contains(set))
{
hashSet.Add(set);
result.Add(set.List);
}
}
}
}
return(result);
}
private List <List <int> > ThreeSum(int[] nums, int target, int ignoreIndex)
{
var result = new List <List <int> >();
var hashSet = new HashSet <NumSet>();
for (int i = 0; i < nums.Length; i++)
{
if (i == ignoreIndex)
{
continue;
}
var twoSum = TwoSum(nums, target - nums[i], i, ignoreIndex);
if (twoSum.Count > 0)
{
foreach (var duplet in twoSum)
{
duplet.Add(nums[i]);
var set = new NumSet(duplet);
if (!hashSet.Contains(set))
{
hashSet.Add(set);
result.Add(set.List);
}
}
}
}
return(result);
}
private List <List <int> > TwoSum(int[] nums, int target, int ignoreIndex1, int ignoreIndex2)
{
var result = new List <List <int> >();
var hashSet = new HashSet <NumSet>();
var dict = new Dictionary <int, List <int> >();
for (int i = 0; i < nums.Length; i++)
{
if (i == ignoreIndex1 || i == ignoreIndex2)
{
continue;
}
if (dict.ContainsKey(nums[i]))
{
dict[nums[i]].Add(i);
}
else
{
dict.Add(nums[i], new List <int>()
{
i
});
}
}
for (int i = 0; i < nums.Length; i++)
{
if (i == ignoreIndex1 || i == ignoreIndex2)
{
continue;
}
int key = target - nums[i];
if (dict.ContainsKey(key))
{
foreach (int index in dict[key].Where(v => v != i))
{
var set = new NumSet(new List <int>()
{
nums[i], nums[index]
});
if (!hashSet.Contains(set))
{
hashSet.Add(set);
result.Add(set.List);
}
}
}
}
return(result);
}
