18 4Sum

链接：https://leetcode.com/problems/4sum/
问题描述：
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)

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Hide Similar Problems (M) Two Sum (M) 3Sum

这一题和3sum类似，思想也是从3sum中借鉴过来的http://blog.csdn.net/efergrehbtrj/article/details/46855331。需要注意的是，这一题返回的结果需要去除重复的。c++容器set可以用来很好的解决这个问题。

class Solution {
public:
    vector<vector<int>> fourSum(vector<int>& nums, int target) {

        vector<vector<int>> result;
        int p1,p2;

        sort(nums.begin(),nums.end());
        for(int i=0;i<nums.size();i++)
        {
            for(int j=i+1;j<nums.size();j++)
            {
               p1=j+1;
               p2=nums.size()-1;
               while(p1<p2)
               {
                   if(nums[i]+nums[j]+nums[p1]+nums[p2]==target)
                   {
                      vector<int> temp;
                      temp.push_back(nums[i]);
                      temp.push_back(nums[j]);
                      temp.push_back(nums[p1]);
                      temp.push_back(nums[p2]);
                      result.push_back(temp);
                      p1++;
                      p2--;
                   }
                   else if(nums[i]+nums[j]+nums[p1]+nums[p2]<target)
                      p1++;
                   else
                       p2--;
               }   
            }
        }
        set<vector<int>> si(result.begin(),result.end());
        vector<vector<int>> res(si.begin(),si.end());

        return res;
    }
};