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4Sum
Given an array S of n integers, are there elements a, b, c, and d in S such that a + b + c + d = target? Find all unique quadruplets in the array which gives the sum of target.
Note:
- Elements in a quadruplet (a,b,c,d) must be in non-descending order. (ie, a ≤ b ≤ c ≤ d)
- The solution set must not contain duplicate quadruplets.
For example, given array S = {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> &num, int target)
{
vector<vector<int>> result;
if(num.size()<4) return result;
sort(num.begin(),num.end());
for(int i1=0;i1<num.size();i1++)
{
if(i1>0 && num[i1]==num[i1-1])
continue;
for(int i2=i1+1;i2<num.size();i2++)
{
if(i2>i1+1 && num[i2]==num[i2-1])
continue;
int i3=i2+1;
int i4=num.size()-1;
while(i3<num.size() && i4<num.size() && i3<i4)
{
if(num[i3]+num[i4]==target-num[i1]-num[i2])
{
vector<int> v;
v.push_back(num[i1]);v.push_back(num[i2]);v.push_back(num[i3]);v.push_back(num[i4]);
result.push_back(v);
do
{
i3++;
}while(i3<num.size()&& num[i3]==num[i3-1]);
do
{
i4--;
}while(i4>i3 && num[i4]==num[i4+1]);
}
else if(num[i3]+num[i4]<target-num[i1]-num[i2])
i3++;
else i4--;
}
}
}
return result;
}
};
public:
vector<vector<int>> fourSum(vector<int> &num, int target)
{
vector<vector<int>> result;
if(num.size()<4) return result;
sort(num.begin(),num.end());
for(int i1=0;i1<num.size();i1++)
{
if(i1>0 && num[i1]==num[i1-1])
continue;
for(int i2=i1+1;i2<num.size();i2++)
{
if(i2>i1+1 && num[i2]==num[i2-1])
continue;
int i3=i2+1;
int i4=num.size()-1;
while(i3<num.size() && i4<num.size() && i3<i4)
{
if(num[i3]+num[i4]==target-num[i1]-num[i2])
{
vector<int> v;
v.push_back(num[i1]);v.push_back(num[i2]);v.push_back(num[i3]);v.push_back(num[i4]);
result.push_back(v);
do
{
i3++;
}while(i3<num.size()&& num[i3]==num[i3-1]);
do
{
i4--;
}while(i4>i3 && num[i4]==num[i4+1]);
}
else if(num[i3]+num[i4]<target-num[i1]-num[i2])
i3++;
else i4--;
}
}
}
return result;
}
};
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