Leetcode no. 18

18. 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)

public class Solution {
    public List<List<Integer>> fourSum(int[] nums, int target) {
        List<List<Integer>> list= new LinkedList<>();
        if (nums.length<4) return list;
        Arrays.sort(nums);
        for (int i = 0; i < nums.length-3; i++) {
            if (i>0 && nums[i]== nums[i-1]) continue;
            for (int j = i+1; j < nums.length-2; j++) {
                if (j>i+1 && nums[j]== nums[j-1]) continue;
                int subtarget= target-nums[i]-nums[j];
                int m= j+1, n= nums.length-1;
                while (m<n){
                    if (nums[m]+nums[n]==subtarget) {
                        list.add(Arrays.asList(nums[i], nums[j], nums[m], nums[n]));
                        while (m < n && nums[m] == nums[m + 1]) m++;
                        while (m < n && nums[n] == nums[n - 1]) n--;
                        m++; n--;
                    } else if (nums[m]+nums[n]<subtarget) m++;
                    else n--;
                }
            }
        }
        return list;
    }
}