_18.java

package com.fishercoder.solutions;

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

public class _18 {

    public static class Solution1 {
        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 - 3; i++) {
                if (i > 0 && nums[i - 1] == nums[i]) {
                    continue;
                }
                for (int j = i + 1; j < nums.length - 2; j++) {
                    if (j > i + 1 && nums[j - 1] == nums[j]) {
                        continue;
                    }
                    int left = j + 1;
                    int right = nums.length - 1;
                    while (left < right) {
                        int sum = nums[i] + nums[j] + nums[left] + nums[right];
                        if (sum == target) {
                            result.add(Arrays.asList(nums[i], nums[j], nums[left], nums[right]));
                            while (left + 1 < right && nums[left] == nums[left + 1]) {
                                left++;
                            }
                            while (right - 1 > left && nums[right] == nums[right - 1]) {
                                right--;
                            }
                            left++;
                            right--;
                        } else if (sum > target) {
                            right--;
                        } else {
                            left++;
                        }
                    }
                }
            }
            return result;
        }
    }

    public static class Solution2 {
        public List<List<Integer>> fourSum(int[] nums, int target) {
            Arrays.sort(nums);
            Set<List<Integer>> set = new HashSet<>();
            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 p = k + 1; p < nums.length; p++) {
                            int sum = nums[i] + nums[j] + nums[k] + nums[p];
                            if (sum == target) {
                                set.add(Arrays.asList(nums[i], nums[j], nums[k], nums[p]));
                            }
                        }
                    }
                }
            }
            List<List<Integer>> result = new ArrayList<>();
            for (List<Integer> each : set) {
                result.add(each);
            }
            return result;
        }
    }
}