leetcode-----18. 四数之和

• 链接：https://leetcode-cn.com/problems/4sum/

代码

class Solution {
    public List<List<Integer>> fourSum(int[] nums, int target) {
        List<List<Integer>> ans = new LinkedList<>();
        if (nums == null || nums.length < 4) return ans;

        Arrays.sort(nums);

        int len =  nums.length;

        for (int i = 0; i < len - 3; i++) {
            if (i > 0 && nums[i] == nums[i - 1]) continue;
            for (int j = len - 1; j > i + 1; j--) {
                int l = i + 1, r = j - 1;
                while (l < r) {
                    int sum = nums[i] + nums[l] + nums[r] + nums[j];
                    if (sum == target) {
                        List<Integer> list = Arrays.asList(nums[i], nums[l], nums[r], nums[j]);
                        if (!ans.contains(list)) {
                            ans.add(list);
                        }
                        while (l < r && nums[r - 1] == nums[r]) r--; 
                        while (l < r && nums[l + 1] == nums[l]) l++; 
                        l++; r--;
                    }
                    if (sum > target) r--;
                    if (sum < target) l++;
                }
            }
        }
        return ans;
    }
}