_18.java

package com.fishercoder.solutions;

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

/**
 * 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]
 ]*/

public class _18 {

    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;
    }

}