Poison

18. 4Sum

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class Solution {
    public List<List<Integer>> fourSum(int[] nums, int target) {
        Arrays.sort(nums);

        List<List<Integer>> resultList = new ArrayList<>();

        for (int i = 0; i < nums.length - 3; i++) {
            if (i > 0 && nums[i] == nums[i - 1]) {
                continue;
            }
            if ((long) nums[i] + nums[i + 1] + nums[i + 2] + nums[i + 3] > target) {
                break;
            }
            if ((long) nums[i] + nums[nums.length - 3] + nums[nums.length - 2] + nums[nums.length - 1] < target) {
                continue; // 注意此处是 continue
            }

            for (int j = i + 1; j < nums.length - 2; j++) {
                if (j > i + 1 && nums[j] == nums[j - 1]) {
                    continue;
                }
                if ((long) nums[i] + nums[j] + nums[j + 1] + nums[j + 2] > target) {
                    break;
                }
                if ((long) nums[i] + nums[j] + nums[nums.length - 2] + nums[nums.length - 1] < target) {
                    continue; // 注意此处是 continue
                }
                
                int k = j + 1, l = nums.length - 1;
                while (k < l) {
                    long fourSum = (long) nums[i] + nums[j] + nums[k] + nums[l]; // 注意此处需要显式转型，避免加法计算时溢出
                    if (fourSum > target) {
                        l--;
                    } else if (fourSum < target) {
                        k++;
                    } else {
                        resultList.add(Arrays.asList(nums[i], nums[j], nums[k++], nums[l--]));
                        while (k < l && nums[k] == nums[k - 1]) {
                            k++;
                        }
                        while (k < l && nums[l] == nums[l + 1]) {
                            l--;
                        }
                    }
                }
            }
        }

        return resultList;
    }
}
Reference

18. 4Sum