【leetcode】4Sum

4Sum

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)

 

与3Sum类似，只是我们现在需要遍历两个元素，a，b，然后在利用两个指针的方法求c，d

 

 1 class Solution {

 2 public:

 3     vector<vector<int> > fourSum(vector<int> &num, int target) {

 4        

 5         int n=num.size();

 6         int i,j,k,l;

 7         int a,b,c,d;

 8         sort(num.begin(),num.end());

 9         vector<vector<int> > result;

10         for(int i=0;i<n-3;i++)

11         {

12             for(int j=i+1;j<n-2;j++)

13             {

14                 k=j+1;

15                 l=n-1;

16  

17                 while(k<l)

18                 {

19                     a=num[i];

20                     if(i>0&&num[i]==num[i-1])

21                     {

22                         break;

23                     }

24                    

25                     b=num[j];

26                     if(j>i+1&&num[j]==num[j-1])

27                     {

28                         break;

29                     }

30                    

31                     c=num[k];

32                     if(k>j+1&&num[k]==num[k-1])

33                     {

34                         k++;

35                         continue;

36                     }

37                    

38                     d=num[l];

39                     if(l<n-1&&num[l]==num[l+1])

40                     {

41                         l--;

42                         continue;

43                     }

44                    

45                     int sum=a+b+c+d;

46                    

47                     if(sum<target)

48                     {

49                         k++;

50                     }

51                     else if(sum>target)

52                     {

53                         l--;

54                     }

55                     else

56                     {

57                         vector<int> tmp(4);

58                         tmp[0]=a;

59                         tmp[1]=b;

60                         tmp[2]=c;

61                         tmp[3]=d;

62                         result.push_back(tmp);

63                         k++;

64                         l--;

65                     }

66                 }

67             }

68         }

69         return result;

70     }

71 };