FourSum.java

package com.company;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

public class FourSum {

    /**
     * Given array nums = [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]
     * ]
     */
    public List<List<Integer>> fourSum(int[] nums, int target) {
        List<List<Integer>> resultList = new ArrayList<>();
        if (nums == null || nums.length < 4) return resultList;
        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 start = j + 1;
                int end = nums.length - 1;
                while (start < end) {
                    int sum = nums[i] + nums[j] + nums[start] + nums[end];
                    if (sum == target) {
                        resultList.add(Arrays.asList(nums[i], nums[j], nums[start], nums[end]));
                        start++;
                        end--;
                        while (start < end && nums[start] == nums[start - 1]) start++;
                        while (start < end && nums[end] == nums[end + 1]) end--;
                    } else if (sum < target) start++;
                    else end--;
                }
            }
        }
        return resultList;
    }
}