LeetCode -- 4Sum

题目描述：


Given an array S of n integers, are there elements a, b, c, and d in S such that a + b + c + d = target? Find all unique quadruplets in the array which gives the sum of target.


Note:
Elements in a quadruplet (a,b,c,d) must be in non-descending order. (ie, a ≤ b ≤ c ≤ d)
The solution set must not contain duplicate quadruplets.
    For example, given array S = {1 0 -1 0 -2 2}, and target = 0.


    A solution set is:
    (-1,  0, 0, 1)
    (-2, -1, 1, 2)
    (-2,  0, 0, 2)


在数组中找到四个数和=target。


和3Sum思路一模一样，就是外面多加了一层循环。


实现代码：

public class Solution {    public IList<IList<int>> FourSum(int[] nums, int target)     {        if(nums == null || nums.Length < 4){    return new List<IList<int>>();    }        var dic = new Dictionary<string, List<int>>();    var list = nums.OrderBy(x=>x).ToList();        var len = list.Count;    for(var j = 0;j < len ; j++)    {    var d = list[j];    for (var i = 0 ;i <= len - 3 ;i++)    {    var a = list[i];    var start = i+1;    var end = len-1;    if(i == j){    continue;    }        while (start < end)    {    if(start == j){    start ++;     continue;    }    if(end == j){    end --;    continue;    }        var b = list[start];    var c = list[end];        if (a+b+c+d == target) {        var v = new List<int>(){a,b,c,d}.OrderBy(x=>x).ToList();    var k = string.Join(",",v);    if(!dic.ContainsKey(k)){    dic.Add(k,v);    }    start ++;    end --;    }    else if (a+b+c+d > target){    end --;    }    else{    start ++;    }    }    }    }            var ret = new List<IList<int>>();    foreach(var kv in dic){    ret.Add(kv.Value);    }        return ret;    }        }