leetCode 18. 4Sum

Given an array nums of n integers and an integer target, are there elements a, b, c, and d in nums 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.

Example:

Given array nums = [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]
]

原本想着将4Sum转化为两个2Sum，做到一半发现这两个2Sum会有重叠，无奈放弃，看了视频教程是将4sum转化为3sum然后再转化为2sum。吐血

 

class Solution {
    public boolean isSame(List<Integer> a,List<Integer> b){
        for (int i = 0; i < 4; i++) {
            int aa = a.get(i);
            int bb = b.get(i);
            if (aa != bb)
                return false;
        }
        return true;
    }
    public List<List<Integer>> fourSum(int[] nums, int target) {
        List<List<Integer>> result = new ArrayList<>();
        Arrays.sort(nums);
        for (int i = 0; i < nums.length - 3; i++) {
            if (i == 0 || nums[i] != nums[i-1]){//去重
                //化为3Sum
                for (int j = i + 1; j < nums.length - 2; j++) {
                    if (j == i + 1 || nums[j] != nums[j - 1]){//去重
                        //化为2Sum
                        int newTarget = target - nums[i] - nums[j];
                        int low = j+1,high=nums.length-1;
                        while (low < high){
                            int sum = nums[low]+nums[high];
                            if (sum > newTarget){
                                --high;
                            }else if (sum < newTarget){
                                ++low;
                            }else{
                                if (result.size() > 0) {
                                    List<Integer> l = result.get(result.size() - 1);
                                    if (!isSame(l,Arrays.asList(nums[i], nums[j], nums[low], nums[high]))) {
                                        result.add(Arrays.asList(nums[i], nums[j], nums[low], nums[high]));
//                                        System.out.println(nums[i] + "," + nums[j] + "," + nums[low] + "," + nums[high]);
                                    }
                                }else {
                                    result.add(Arrays.asList(nums[i], nums[j], nums[low], nums[high]));
//                                    System.out.println(nums[i] + "," + nums[j] + "," + nums[low] + "," + nums[high]);
                                }
                                ++low;
                            }
                        }
                    }
                }
            }
        }
        return result;
    }
}