4Sum

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

Note:

The solution set must not contain duplicate quadruplets.

Example:

Given array nums = [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]
]

class Solution {
public:
    vector<vector<int>> fourSum(vector<int>& nums, int target) {
        vector<vector<int> > res;
        vector<int> tempres;
        if(nums.empty())
            return res;
        if(nums.size()<=3)
            return res;
        sort(nums.begin(), nums.end());
        for(int j=0; j<nums.size()-3; j++)
        {
            if (j > 0 && nums[j] == nums[j - 1]) continue;   //这一步很关键
            for(int i=j+1; i<nums.size()-2; i++)
            {
                if(i>j+1 && nums[i] == nums[i - 1]) continue;  //这一步也很关键
                int start =i+1;
                int end = nums.size()-1;
                int sum = nums[i];
                while(start<end)
                {
                    if((nums[start]+nums[end]+nums[i]+nums[j])==target)
                    {
                        tempres.push_back(nums[j]);
                        tempres.push_back(nums[i]);
                        tempres.push_back(nums[start]);
                        tempres.push_back(nums[end]);
                        res.push_back(tempres);
                        while( start< end && nums[start] == nums[start + 1]) ++start;
                        while (start< end && nums[end] == nums[end - 1]) --end;
                        tempres.clear();
                        ++start;
                        --end;
                    
                    }else if((nums[start]+nums[end]+nums[i]+nums[j])>target)
                    {
                        --end;
                    }else
                    {
                        ++start;
                    }
                }
            }
                
        }
        return res;
    }
};