leetcode 日记 4sum java

整体思路同之前的一样，依然采取降低维度的方式进行

public List<List<Integer>> solution(int nums[], int target) {
        List<List<Integer>> result = new ArrayList<>();
        if((nums.length<4)||(nums==null))
        {
            return result;
        }
        Arrays.sort(nums);
        if ((4*nums[0]>target)||(4*nums[nums.length-1]<target))
        {
            return result;
        }//上面两个队特例的快速结束一定要加
        int N = nums.length;
        for (int i = 0; i < N; i++) {
            int[] temnum = new int[N - 1];
            System.arraycopy(nums, 0, temnum, 0, i);
            System.arraycopy(nums, i + 1, temnum, i, N - i - 1);
            result = threeSum(temnum, target - nums[i], nums[i], result);
        }
        return result;
    }


    public List<List<Integer>> threeSum(int[] nums, int target, int num,List<List<Integer>> result) {
        int sum;
        for (int i = 0; i < nums.length - 2; i++) {
            int start = i + 1, end = nums.length - 1;
            while (start < end) {
                sum = nums[i] + nums[start] + nums[end];
                if (sum < target) {
                    start++;
                    continue;
                }
                if (sum > target) {
                    end--;
                    continue;
                }
                //下面的部分为对每一结果内部进行进行排序，这样在去重时方便
                if (sum == target) {
                    List<Integer> list = new ArrayList<>();
                    if(num<nums[i])
                        list.add(num);
                    list.add(nums[i]);
                    if(num>=nums[i] && num<nums[start])
                        list.add(num);
                    list.add(nums[start]);
                    if ( num>=nums[start] &&num<nums[end])
                        list.add(num);
                    list.add(nums[end]);
                    if (num>=nums[end])
                        list.add(num);
                    start++;
                    end--;
                    if (result.contains(list))
                        continue;
                    result.add(list);
                }
            }
        }
        return result;
    }                

整体速度在leetcode上大概为50%