Description

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]
]

Solution

Four-pointers, O(n^3) time

先对数组进行排序，注意去重。跟3Sum的思路一样。

class Solution {
    public List<List<Integer>> fourSum(int[] nums, int target) {
        List<List<Integer>> quadruplets = new ArrayList();
        if (nums == null || nums.length < 4) return quadruplets;
        
        Arrays.sort(nums);
        int i = 0;
        int n = nums.length;
        
        while (i < n - 3) {
            int j = i + 1;
            while (j < n - 2) {
                int k = j + 1;
                int l = n - 1;
                while (k < l) {
                    int sum = nums[i] + nums[j] + nums[k] + nums[l];
                    if (sum < target) {
                        ++k;
                    } else if (sum > target) {
                        --l;
                    } else {
                        quadruplets.add(
                            Arrays.asList(nums[i], nums[j], nums[k], nums[l]));
                        while (++k < l && nums[k] == nums[k - 1]) {};
                        while (k < --l && nums[l] == nums[l + 1]) {};
                    }
                }
                while (++j < n - 2 && nums[j] == nums[j - 1]) {};
            }
            while (++i < n - 3 && nums[i] == nums[i - 1]) {};
        }
        
        return quadruplets;
    }
}