18 4Sum

和3sum 相似 ，比3sum多一层循环。

public class Solution {
    public ArrayList<ArrayList<Integer>> fourSum(int[] nums, int target) {
        ArrayList<ArrayList<Integer>> res = new ArrayList<ArrayList<Integer>>();
        if (nums == null || nums.length < 3) {
            return res;
        }
        Arrays.sort(nums);
        for (int i = 0; i< nums.length - 3; i++) {
            if (i != 0 && nums[i] == nums[i-1]) {
                continue;
            }
            
            for (int j = i+1; j<nums.length - 2; j++) {
                if (j != i+1 && nums[j] == nums[j-1]) {
                    continue;
                }
                
                int left = j+1;
                int right = nums.length - 1;
                while (left < right) {
                    int sum = nums[left] + nums[right] + nums[i] + nums[j];
                    if (sum < target) {
                        left++;
                    } else if (sum > target) {
                        right--;
                    } else {
                        ArrayList<Integer> tmp = new ArrayList<Integer>();
                        tmp.add(nums[i]);
                        tmp.add(nums[j]);
                        tmp.add(nums[left]);
                        tmp.add(nums[right]);
                        res.add(tmp);
                        left++;
                        right--;
                        while (left < right && nums[left] == nums[left - 1]) {
                            left++;
                        }
                        while (left < right && nums[right] == nums[right +1]) {
                            right--;
                        }
                    }
                }
            }
        }
        return res;
    }
}