FourSum.java

import java.util.*;

/**
 * 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.
 * <p>
 * Note: The solution set must not contain duplicate quadruplets.
 * <p>
 * For example, given array S = [1, 0, -1, 0, -2, 2], and target = 0.
 * <p>
 * A solution set is:
 * [
 * [-1,  0, 0, 1],
 * [-2, -1, 1, 2],
 * [-2,  0, 0, 2]
 * ]
 * <p>
 * Accepted.
 */
public class FourSum {

    // Time Limit Exceeded.
    /*public List<List<Integer>> fourSum(int[] nums, int target) {
        Set<List<Integer>> set = new HashSet<>();
        Arrays.sort(nums);

        for (int i = 0; i < nums.length - 3; i++) {
            for (int j = i + 1; j < nums.length - 2; j++) {
                for (int k = j + 1; k < nums.length - 1; k++) {
                    for (int m = k + 1; m < nums.length; m++) {
                        int sum = nums[i] + nums[j] + nums[k] + nums[m];
                        if (sum > target) {
                            continue;
                        }
                        if (sum == target) {
                            List<Integer> tmp = new ArrayList<>(4);
                            tmp.add(nums[i]);
                            tmp.add(nums[j]);
                            tmp.add(nums[k]);
                            tmp.add(nums[m]);
                            set.add(tmp);
                        }
                    }
                }
            }
        }

        return new ArrayList<>(set);
    }*/

    public List<List<Integer>> fourSum(int[] nums, int target) {
        List<List<Integer>> results = new ArrayList<>();
        Arrays.sort(nums);

        for (int i = 0; i < nums.length - 3; i++) {
            for (int j = i + 1; j < nums.length - 2; j++) {
                int left = j + 1, right = nums.length - 1;
                while (left < right) {
                    int sum = nums[i] + nums[j] + nums[left] + nums[right];
                    if (sum == target) {
                        List<Integer> tmp = new ArrayList<>(4);
                        tmp.add(nums[i]);
                        tmp.add(nums[j]);
                        tmp.add(nums[left]);
                        tmp.add(nums[right]);
                        if (!results.contains(tmp)) {
                            results.add(tmp);
                        }
                        left++;
                        right--;
                    } else if (sum < target) {
                        left++;
                    } else {
                        right--;
                    }
                }
            }
        }

        return results;
    }

}