c# leetcode 18. 四数之和

public class Solution {
    private int _len;
    public IList<IList<int>> FourSum(int[] nums, int target) {
         _len = nums.Length;
        nums = nums.OrderBy(n => n).ToArray();
        return KSum(nums, target, 4, 0);
    }
    private IList<IList<int>> KSum(int[] nums, int target, int k, int index)
    {
        var res = new List<IList<int>>();

        if (index >= _len) return res;

        if (k == 2)
        {
            int i = index, j = _len - 1;
            while (i < j)
            {
                // Find a pair
                if (target - nums[i] == nums[j])
                {
                    res.Add(new List<int> { nums[i], target - nums[i] });

                    // Skip duplication
                    while (i < j && nums[i] == nums[i + 1]) i++;
                    while (i < j && nums[j - 1] == nums[j]) j--;
                    i++;
                    j--;
                }
                // Move left bound
                else if (target - nums[i] > nums[j]) i++;
                // Move right bound
                else j--;
            }
        }
        else
        {
            for (var i = index; i < _len - k + 1; i++)
            {
                // Use current number to reduce K Sum into K-1 Sum
                var temp = KSum(nums, target - nums[i], k - 1, i + 1);
                if (temp.Any())
                {
                    // Add previous results
                    foreach (var t in temp)
                    {
                        t.Add(nums[i]);
                    }
                    res.AddRange(temp);
                }

                // Skip duplicated numbers
                while (i < _len - 1 && nums[i] == nums[i + 1]) i++;
            }

        }

        return res;
    }
}

答案二：

public IList<IList<int>> FourSum(int[] nums, int target) {
          
     var result = new List<IList<int>>();
        
        var map = new Dictionary<int, List<IList<int>>>();
        
        for(int i = 0; i < nums.Length; i++)
        {
            for(int j = i+1; j< nums.Length; j++)
            {
                int sum = nums[i] + nums[j];
                
                var pair = new List<int>(){i,j};
                
                if(map.ContainsKey(sum))
                {
                    map[sum].Add(pair);
                }
                else
                {
                    map.Add(sum, new List<IList<int>>() {pair});
                }
            }
        }
        
        var set = new HashSet<string>();
        
        foreach(var key in map.Keys)
        {
            if(map.ContainsKey(target - key))
            {
                if(target - key == key && map[key].Count == 1)
                {
                    continue;
                }
                
                foreach(var p1 in map[key])
                {
                    foreach(var p2 in map[target-key])
                    {                      
                        if(p1.Contains(p2[0]) || p1.Contains(p2[1]) || p2.Contains(p1[0]) || p2.Contains(p1[1]))
                        {
                            continue;
                        }
                        
                        var tempList = new List<int>();
                        
                        tempList.Add(nums[p1[0]]);
                        tempList.Add(nums[p1[1]]);
                        tempList.Add(nums[p2[0]]);
                        tempList.Add(nums[p2[1]]);
                        
                        tempList.Sort();
                        
                        var checkKey = string.Join("", tempList.ToArray());
                        
                        if(!set.Contains(checkKey))
                        {
                            result.Add(tempList);
                            set.Add(checkKey);
                        }                      
                    }
                }
            }
        }
        
        return result;
    }