Solution.java

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

/**
 * User: Changle
 * Date: 2018-03-11 10:21
 * Source: https://leetcode.com/problems/4sum/description/
 */

/**
 * 排序，然后确定两个数，然后使用对撞指针移动另外两个数
 *
 * 时间复杂度: O(n^3)
 * 空间复杂度: O(n)
 */
public class Solution {

    public List<List<Integer>> fourSum(int[] nums, int target) {
        List<List<Integer>> res = new ArrayList<>();
        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 left = j + 1;
                int right = nums.length - 1;
                while (left < right) {
                    if (nums[i] + nums[j] + nums[left] + nums[right] < target) {
                        while (left < right && nums[left] == nums[left + 1]) {
                            left++;
                        }
                        left++;
                    } else if (nums[i] + nums[j] + nums[left] + nums[right] > target) {
                        while (left < right && nums[right] == nums[right - 1]) {
                            right--;
                        }
                        right--;
                    } else {
                        res.add(Arrays.asList(nums[i], nums[j], nums[left], nums[right]));
                        while (left < right && nums[left] == nums[left + 1]) {
                            left++;
                        }
                        left++;
                        while (left < right && nums[right] == nums[right - 1]) {
                            right--;
                        }
                        right--;
                    }
                }
            }
        }

        return res;
    }

}