two sum&&three sum&&four sum&&three sum closet

two sum

class Solution {
public:
    vector twoSum(vector& nums, int target) {
        vector rec(2,-1);
        unordered_map m;
        int n = nums.size();
        if(n<2)
          return rec;
        for(int i=0;i

3 sum

class Solution {
public:
    vector> threeSum(vector& nums) {
        vector> result;
        int n = nums.size();
        sort(nums.begin(),nums.end());
        if(n<=2)
          return result;
        
        for(int i=0;i rec;
                   rec.push_back(nums[i]);
                   rec.push_back(nums[j]);
                   rec.push_back(nums[k]);
                   result.push_back(rec);
                   j++;
                   k--;
                   while(j

3Sum Closest

class Solution {
public:
    int threeSumClosest(vector& nums, int target) {
        int n = nums.size();
        sort(nums.begin(),nums.end());
        int last = -1;
        int minClosest = INT_MAX;
        
        for(int i=0;itarget)
                  k--;
                else if(sum==target)
                {
                  j++;
                  k--;
                }
                if(abs(target-sum)

4 sum

class Solution {
public:
    vector> fourSum(vector& nums, int target) {
        vector> result;
        int n = nums.size();
        if(n<=3)
          return result;
        sort(nums.begin(),nums.end());
        for(int i=0;i rec;
                        rec.push_back(nums[i]);
                        rec.push_back(nums[j]);
                        rec.push_back(nums[k]);
                        rec.push_back(nums[l]);
                        result.push_back(rec);
                        k++;
                        l--;
                        while(k

利用set来实现：

3 sum

class Solution {
public:
    vector> threeSum(vector& nums) {
        vector> result;
        set> mSet;
        int n = nums.size();
        sort(nums.begin(),nums.end());

        if(n<=2)
          return result;
        
        for(int i=0;i rec;
                   rec.push_back(nums[i]);
                   rec.push_back(nums[j]);
                   rec.push_back(nums[k]);
                   mSet.insert(rec);
                   j++;
                   k--;
                }
                else if(nums[i]+nums[j]+nums[k]<0)
                   j++;
                else 
                   k--;
            }

        }
        return vector>(mSet.begin(),mSet.end()); //注意此种写法！！！
    }
};

4 sum

class Solution {
public:
    vector> fourSum(vector& nums, int target) {
        vector> result;
        int n = nums.size();
        if(n<=3)
          return result;
        set> mSet;
        sort(nums.begin(),nums.end());
        for(int i=0;i rec;
                        rec.push_back(nums[i]);
                        rec.push_back(nums[j]);
                        rec.push_back(nums[k]);
                        rec.push_back(nums[l]);
                        mSet.insert(rec);
                        k++;
                        l--;
                    }
                    else if(nums[k] + nums[l] < sum)
                    {
                        k++;   
                    }
                    else
                    {
                        l--;
                    }
                }
            }
        }
        return vector>(mSet.begin(),mSet.end());//注意此种写法！！！
    }
};