leetcode解题方案--018--4 sum

题目

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]
]

分析

没什么可分析的 直接看本博客的3sum即可

 public static List<List<Integer>> fourSum(int[] nums, int target) {        Arrays.sort(nums);        List<List<Integer>> list = new LinkedList<List<Integer>>();        for (int i = 0; i < nums.length - 3; i++) {            if (i == 0 || (i > 0 && nums[i] != nums[i - 1])) {                for (int j = i + 1; j < nums.length - 2; j++) {                    if (j==i+1 || (j>i+1 && nums[j] != nums[j - 1])) {                        int sum = target - nums[i] - nums[j];                        int lo = j + 1;                        int hi = nums.length - 1;                        while (lo < hi) {                            if (nums[lo] + nums[hi] == sum) {                                list.add(Arrays.asList(nums[i], nums[j], nums[lo], nums[hi]));                                while (lo < hi && nums[lo] == nums[lo + 1]) {                                    lo++;                                }                                while (lo < hi && nums[hi] == nums[hi - 1]) {                                    hi--;                                }                                ;                                lo++;                                hi--;                            } else if (nums[lo] + nums[hi] > sum) {                                hi--;                            } else if (nums[lo] + nums[hi] < sum) {                                lo++;                            }                        }                    }                }            }        }        return list;    }