leetcode18. 四数之和

class Solution {
    public List<List<Integer>> fourSum(int[] nums, int target) {
        if(nums.length < 4) {
			return new ArrayList<>();
		}
		Arrays.sort(nums);
		//??set??
		Set<List<Integer>> set = new HashSet<>();
		for(int a = 0; a < nums.length-3; a ++) {
			//????
			if(nums[a] + nums[nums.length-1] +nums[nums.length-2]+nums[nums.length-3] < target) {
				continue;
			}
			if(nums[a] + nums[a+1] +nums[a+2]+nums[a+3] > target) {
				break;
			}
			if(a>0 && nums[a] == nums[a-1]) {
				continue;
			}
			for(int b = nums.length-1; b > a; b--) {
				int c = a+1, d = b-1;
				while(c < d) {
					int tmp = nums[a] + nums[b] + nums[c] + nums[d];
					if(tmp == target) {
						List<Integer> list = new ArrayList<>();
						list.add(nums[a]);list.add(nums[b]);list.add(nums[c]);list.add(nums[d]);
						set.add(list);
						c++;d--;
						continue;
					}
					else if(tmp < target) {
						c++;
					}
					else {
						d--;
					}
				}
			}
		}
		return new ArrayList<>(set);
    }
}