https://oj.leetcode.com/problems/4sum/
http://www.programcreek.com/2013/02/leetcode-4sum-java/
public class Solution {
public List<List<Integer>> fourSum(int[] num, int target)
{
// Solution A
return fourSum_Sort(num, target);
}
/////////////////////////////////
// Solution A: Sort
//
// General solution for k-sum
private List<List<Integer>> fourSum_Sort(int[] num, int target)
{
if (num == null || num.length < 4)
return Collections.emptyList(); // Invalid input
// Sort
Arrays.sort(num);
int len = num.length;
Set<List<Integer>> result = new HashSet<>();
// O(n^3)
for (int i = 0 ; i < len ; i ++)
{
for (int j = i + 1 ; j < len ; j ++)
{
int m = j + 1;
int n = len - 1;
while (m < n)
{
int sum = num[i] + num[j] + num[m] + num[n];
if (sum == target)
{
// Solution find
List<Integer> r = tolist(num[i], num[j], num[m], num[n]);
Collections.sort(r);
result.add(r);
m ++;
n --;
}
else if (sum < target)
{
m ++;
}
else
{
n --;
}
}
}
}
return new ArrayList<>(result);
}
private List<Integer> tolist(int... nums)
{
List<Integer> toReturn = new ArrayList<>(nums.length);
for (int i : nums)
toReturn.add(i);
return toReturn;
}
}
