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: 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]
]

Solution:

Same with 3Sum.

Just having one more loop.



Code:

public class Solution {
    public List<List<Integer>> fourSum(int[] nums, int target) {
        List<List<Integer>> result = new ArrayList<>();
        if (nums == null || nums.length == 0) {
            return result;
        }
        Arrays.sort(nums);
        for (int i = 0; i < nums.length; i++) {
            if (i != 0 && nums[i] == nums[i - 1]) {
                continue;
            }
            for (int j = i + 1; j < nums.length; j++) {
                if (j != i + 1 && nums[j] == nums[j - 1]) {
                    continue;
                }
                int lo = j + 1;
                int hi = nums.length - 1;
                while (lo < hi) {
                    if (nums[lo] + nums[hi] + nums[i] + nums[j] == target ) {
                        List<Integer> list = new ArrayList<>();
                        list.add(nums[i]);
                        list.add(nums[j]);
                        list.add(nums[lo]);
                        list.add(nums[hi]);
                        result.add(list);
                        lo++;
                        hi--;
                        while (lo < hi && nums[lo] == nums[lo - 1]) {
                            lo++;
                        }
                        while (lo < hi && nums[hi] == nums[hi + 1]) {
                            hi--;
                        }
                    }
                    else if (nums[lo] + nums[hi] + nums[i] + nums[j] < target) {
                        lo++;
                    }
                    else {
                        hi--;
                    }
                }
            }
        }
        return result;
    }
}