【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

 class Solution {

 public:

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

         int n=num.size();

         int i,j,k,l;

         int a,b,c,d;

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

         vector<vector<int> > result;

         for(int i=;i<n-;i++)

         {

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

             {

                 k=j+;

                 l=n-;

                 while(k<l)

                 {

                     a=num[i];

                     if(i>&&num[i]==num[i-])

                     {

                         break;

                     }

                     b=num[j];

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

                     {

                         break;

                     }

                     c=num[k];

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

                     {

                         k++;

                         continue;

                     }

                     d=num[l];

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

                     {

                         l--;

                         continue;

                     }

                     int sum=a+b+c+d;

                     if(sum<target)

                     {

                         k++;

                     }

                     else if(sum>target)

                     {

                         l--;

                     }

                     else

                     {

                         vector<int> tmp();

                         tmp[]=a;

                         tmp[]=b;

                         tmp[]=c;

                         tmp[]=d;

                         result.push_back(tmp);

                         k++;

                         l--;

                     }

                 }

             }

         }

         return result;

     }

 };

巴特西

【leetcode】4Sum

4Sum

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