【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=0;i<n-3;i++)
        {
            for(int j=i+1;j<n-2;j++)
            {
                k=j+1;
                l=n-1;
 
                while(k<l)
                {
                    a=num[i];
                    if(i>0&&num[i]==num[i-1])
                    {
                        break;
                    }
                   
                    b=num[j];
                    if(j>i+1&&num[j]==num[j-1])
                    {
                        break;
                    }
                   
                    c=num[k];
                    if(k>j+1&&num[k]==num[k-1])
                    {
                        k++;
                        continue;
                    }
                   
                    d=num[l];
                    if(l<n-1&&num[l]==num[l+1])
                    {
                        l--;
                        continue;
                    }
                   
                    int sum=a+b+c+d;
                   
                    if(sum<target)
                    {
                        k++;
                    }
                    else if(sum>target)
                    {
                        l--;
                    }
                    else
                    {
                        vector<int> tmp(4);
                        tmp[0]=a;
                        tmp[1]=b;
                        tmp[2]=c;
                        tmp[3]=d;
                        result.push_back(tmp);
                        k++;
                        l--;
                    }
                }
            }
        }
        return result;
    }
};