[leetcode]18. 4Sum

//用夾逼定理
class Solution {
public:
    vector<vector<int>> fourSum(vector<int>& nums, int target) 
    {
        vector<vector<int>> ret;
        
        sort(nums.begin(),nums.end());//最重要
        
        for(int i=0;i<nums.size();i++)
            for(int j=i+1;j<nums.size();j++)
            {
                int k=j+1;
                int l=nums.size()-1;
                while(k<l)
                {
                    int sum=nums[i]+nums[j]+nums[k]+nums[l];
                    if(sum==target)
                    {
                        vector<int> cur;
                        cur.push_back(nums[i]);
                        cur.push_back(nums[j]);
                        cur.push_back(nums[k]);
                        cur.push_back(nums[l]);
                        
                        if( find(ret.begin(),ret.end(),cur)==ret.end() )//排重
                            ret.push_back(cur);
                        k++;
                        l--;
                    }
                    else if(sum<target)
                    {
                        k++;
                    }
                    else
                    {
                        l--;
                    }
                }
            }
        return ret;
    }
};