leetcode-18 4Sum



问题描述：Givenan 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 containduplicate quadruplets.

   
For example, given array S = {1 0 -1 0 -22}, and target = 0.

   
A solution set is:

   
(-1, 0, 0, 1)

   
(-2, -1, 1, 2)

   
(-2, 0, 0, 2)

问题分析：KSum问题，往下最终转化为基本的2Sum问题，同时注意消除重复值即可

代码：

java解法：
public class Solution {
public List<List<Integer>> fourSum(int[] num, int target) {
List<List<Integer>> result = new ArrayList<List<Integer>>();
if(num == null || num.length <= 3)
return result;

//先对数组进行排序
Arrays.sort(num);

int temp_target = 0;//每一次转化为2Sum问题的目标值
for(int i = 0; i < num.length - 3; i++)
{
//消除重复值
while((i != 0) && (i < num.length - 3) && (num[i] == num[i - 1]))
++ i;
//消除重复值之后一定要注意数组边界的判断，避免出现越界情况
if(i < num.length - 3)
{
for(int j = i + 1; j < num.length - 2; j++)
{
while((j != i + 1) && (j < num.length - 2) && (num[j] == num[j - 1]))
++ j;
if(j < num.length - 2)
{
temp_target = target - num[i] - num[j];
int start = j + 1;
int end = num.length - 1;

while(start < end)
{
int temp_sum = num[start] + num[end];
if(temp_sum < temp_target)
{
++ start;
}
else if(temp_sum > temp_target)
{
-- end;
}
else
{
List<Integer> list = new ArrayList<>();
list.add(num[i]);
list.add(num[j]);
list.add(num[start++]);
list.add(num[end--]);

result.add(list);

//继续往前搜索，并消除相同值
while((start < end) && (num[start] == num[start - 1]))
++ start;
while((start < end) && (num[end] == num[end + 1]))
-- end;
}
}
}
}
}
}
return result;
}
}

class Solution {
public:
vector<vector<int> > fourSum(vector<int> &num, int target) {
// Note: The Solution object is instantiated only once.
vector<vector<int>> res;
int numlen = num.size();
if(num.size()<4) return res;

sort(num.begin(),num.end());
set<vector<int>> tmpres;
for(int i = 0; i < numlen; i++)
{
for(int j = i+1; j < numlen; j++)
{
int begin = j+1;
int end = numlen-1;
while(begin < end)
{
int sum = num[i]+ num[j] + num[begin] + num[end];
if(sum == target)
{
vector<int> tmp;
tmp.push_back(num[i]);
tmp.push_back(num[j]);
tmp.push_back(num[begin]);
tmp.push_back(num[end]);
tmpres.insert(tmp);
begin++;
end--;
}else if(sum<target)
begin++;
else
end--;
}
}
}
set<vector<int>>::iterator it = tmpres.begin();
for(; it != tmpres.end(); it++)
res.push_back(*it);
return res;
}
};