Leetcode 3 sum && 3 sum closet&& 4 sum

 3 sum

a+b+c=0

class Solution {
public:
    vector<vector<int> > threeSum(vector<int> &num) {
        vector<vector<int> >v;  
        vector<int>v1;
        sort(num.begin(),num.end());    
        for(int i=0;i<num.size();i++)
        {
            if(num.size()<3)break;
            if(num[i]>0)break;
            for(int a=i+1,b=num.size()-1;a<b;)
            {  
                int sum=num[i]+num[a]+num[b];
                if(sum==0)
                {
                   v1.clear();
                   v1.push_back(num[i]);
                   v1.push_back(num[a]);
                   v1.push_back(num[b]);
                   if(count(v.begin(),v.end(),v1)==0)
                   v.push_back(v1);
                   a++;b--;
                 }
                 if(sum<0)
                 {
                     a++;
                 }
                 if(sum>0){b--;}         
            }        
        }
        return v;
    }
};

3 sum closet

class Solution {
public:
    int threeSumClosest(vector<int> &num, int target) {
        int n=num.size();
        assert(n>=3);
        sort(num.begin(),num.end());
        int value;
        int sum=num[0]+num[1]+num[2];
        int diff=abs(sum-target);
        int temp=sum;
        for(int i=0;i<n-2;i++)
        {
            int a=i;
            int b=n-1;
            int m;
            value = target-num[a]-num[b];
            while(a<b)
            {    
                m=a+(b-a)/2;           
                  if(value==num[m])return target;  
                  if(abs(num[m]-value)<diff)
                  {
                      diff=abs(num[m]-value);
                      temp=target-value+num[m];
                  }
                  if(num[m]>value)
                  {
                    b=m-1;
                  }
                  else
                  {
                    a=m+1;
                  }         
            }       
        }
        return temp;
    }
};
        

 以上代码有问题，二分查找此处中间值不能等于边界值。

class Solution { 
public: 
    int threeSumClosest(vector<int> &num, int target) { 
        int n=num.size(); 
        assert(n>=3); 
        sort(num.begin(),num.end()); 
        int value; 
        int sum=num[0]+num[1]+num[2]; 
        int diff=abs(sum-target); 
        int temp=sum; 
        int i=0,j=n-1;
        while(i<n-2&&j>1&&i<j-1)
        { 
            int a=i; 
            int b=j; 
            int m; 
            value = target-num[a]-num[b]; 
            while(a<b-1) 
            {     
                m=a+(b-a)/2;
                if(value==num[m])return target;   
                if(abs(num[m]-value)<diff) 
                { 
                    diff=abs(num[m]-value); 
                    temp=target-value+num[m]; 
                } 
                if(num[m]>value) 
                { 
                   b=m; 
                } 
                else
                { 
                   a=m; 
                }          
            } 
            if(value<0)j--;
            else if(value>0) i++;
            else if(value==0&&num[m]>0)j--;
            else i++;
        } 
        return temp; 
    } 
};  

4 sum

定义一对指针，指向两头。再定义一对指针，指向中间的两个元素。加起来看看跟target比较一下。再决定内部指针怎么移动

class Solution {
public:
    vector<vector<int> > fourSum(vector<int> &num, int target) {
        vector<int>v;
        vector<vector<int>>v1;
        int len=num.size();
        if(len<=3)return v1;
        sort(num.begin(),num.end());
        for(int i=0;i<len-3;i++)
        {
            for(int k=len-1;k>i+2;k--)
            {
                int ab=num[i]+num[k];
                int c=target-ab;
                int m=i+1,n=k-1;
                for(;m<n;)
                {
                    int sum=num[m]+num[n];
                    if(sum==c)
                    {
                        v.clear();
                        v.push_back(num[i]);
                        v.push_back(num[m]);
                        v.push_back(num[n]);
                        v.push_back(num[k]);
                        if(count(v1.begin(),v1.end(),v)==0)
                        v1.push_back(v);
                        m++;n--;
                    }
                    else if(sum<c)
                    {
                        m++;
                    }
                    else n--;
                }
            }
        }
        return v1;      
    }
};