leetcode 15. 3sum && 16. 3Sum Closest && 18. 4Sum

class Solution {
public:
vector<vector<int>> threeSum(vector<int>& nums) {
vector<vector<int>>ret;
auto sz=nums.size();
if(sz<3)return ret;
sort(nums.begin(),nums.end());
for(int i=0;i!=sz-2;++i){
if(i>0 && nums[i]==nums[i-1])continue; //此条件判断i是否重合，试想若固定i,并能找出j和k,
//若v[i+1]==v[i],且i+1<j,则产生重合答案.  若找不出答案，则i+1也必然没有结果，所以要跳过
int j=i+1,k=sz-1;
while(j<k){
if(nums[i]+nums[j]+nums[k]==0){
vector<int>v{nums[i],nums[j],nums[k]};
ret.push_back(v);		//当找到目标，自然可以两头缩小再去查找
++j;
while(j<k &&nums[j]==nums[j-1])++j;
--k;
while(j<k && nums[k]==nums[k+1])--k;
}else if(nums[i]+nums[j]+nums[k]>0){	//一头缩小
--k;
}else ++j;
}
}
return ret;
}
};

class Solution {
public:
int threeSumClosest(vector<int>& nums, int target) {
auto sz=nums.size();
if(sz<=3)return accumulate(nums.begin(),nums.end(),0);
sort(nums.begin(),nums.end());
int ret=nums[0]+nums[1]+nums[2],sum=0;
for(int i=0;i!=sz-2;++i){
int j=i+1,k=sz-1;
while(j<k){
sum=nums[i]+nums[j]+nums[k];
if(abs(sum-target)<abs(target-ret))ret=sum;
if(sum==target)return ret;
sum<target? ++j:--k;
}
}
return ret;
}
};

class Solution {
public:
vector<vector<int>> fourSum(vector<int>& nums, int target) {
vector<vector<int>>ret;
auto sz=nums.size();
if(sz<4)return ret;
sort(nums.begin(),nums.end());
for(int i=0;i!=sz-3;++i){
if(i>0 && nums[i]==nums[i-1])continue;
if(nums[i]+nums[i+1]+nums[i+2]+nums[i+3]>target)break;
if(nums[i]+nums[sz-3]+nums[sz-2]+nums[sz-1]<target)continue;
for(int j=i+1;j!=sz-2;++j){
if(j>i+1&&nums[j]==nums[j-1])continue;
if(nums[i]+nums[j]+nums[j+1]+nums[j+2]>target)break;
if(nums[i]+nums[j]+nums[sz-2]+nums[sz-1]<target)continue;
int l=j+1,r=sz-1;
while(l<r){
if(nums[i]+nums[j]+nums[l]+nums[r]<target)++l;
else if(nums[i]+nums[j]+nums[l]+nums[r]>target)--r;
else{
ret.push_back(vector<int>{nums[i],nums[j],nums[l],nums[r]});
do{++l;}while(l<r&&nums[l]==nums[l-1]);
do{--r;} while(l<r && nums[r]==nums[r+1]);
}
}
}
}
return ret;
}
};