三数&四数之和

三数

class Solution {
    public List<List<Integer>> threeSum(int[] nums) {
        Arrays.sort(nums);
        Set<List<Integer>> set = new HashSet<>();
        int left, right;
        int len = nums.length;
        for (int i = 0; i < nums.length - 2; i++) {
            if (i > 0 && nums[i] == nums[i - 1]) continue; // 跳过重复的i
            if (nums[i] + nums[i + 1] + nums[i + 2] > 0) break; // 最小的三个加起来都大于0了，之后的i也不可能，直接退出循环
            if (nums[i] + nums[len - 1] + nums[len - 2] < 0) continue; // nums[i] 加上最大的两个数都小于0，直接跳过这个i
            left = i + 1;
            right = nums.length - 1;
            int sum;
            while (left < right) {
                sum = nums[i] + nums[left] + nums[right];
                if (sum == 0) {
                    set.add(new ArrayList<>(Arrays.asList(nums[i], nums[left], nums[right])));
                    while (left < right && nums[left] == nums[left + 1]) left++;
                    while (left < right && nums[right] == nums[right - 1]) right--;
                    left++;
                    right--;
                } else if (sum < 0) {
                    left++;
                } else if (sum > 0) {
                    right--;
                }
            }
        }
        return new ArrayList<>(set);
    }
}

四数

class Solution {
    public List<List<Integer>> fourSum(int[] nums, int target) {
        Arrays.sort(nums);
        int len = nums.length;
        Set<List<Integer>> set = new HashSet<>();
        int left, right;
        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[len - 1] + nums[len - 2] + nums[len - 3] < target) 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[len - 2] + nums[len - 1] < target) continue;
                left = j + 1;
                right = nums.length - 1;
                while (left < right) {
                    int sum = nums[i] + nums[j] + nums[left] + nums[right];
                    System.out.println("sum :" + sum);
                    if (sum == target) {
                        set.add(Arrays.asList(nums[i], nums[j], nums[left], nums[right]));
                        while (left < right && nums[left] == nums[left + 1]) left++;
                        while (left < right && nums[right] == nums[right - 1]) right--;
                        left++;
                        right--;
                    } else if (sum < target) {
                        left++;
                    } else {
                        right--;
                    }
                }
            }
        }
        return new ArrayList<>(set);
    }
}