ThreeSum.java

//Runtime: 47 ms, faster than 35.83% of Java online submissions for 3Sum.
//Memory Usage: 60.1 MB, less than 32.20% of Java online submissions for 3Sum.
//Two pointers
//Time: O(N * LogN + N * N); Space : O(N + LogN)
/**
 * ThreeSum
 */
public class ThreeSum {
    public List<List<Integer>> threeSum(int[] nums) {
        Set<List<Integer>> resultSet = new HashSet();
        Arrays.sort(nums);

        for (int i = 0; i <= nums.length - 3 && nums[i] <= 0;) {
            int left = i + 1, right = nums.length - 1;
            if (0 - nums[i] - nums[left] < 0)
                break;

            while (left < right) {
                int sum = nums[i] + nums[left] + nums[right];
                if (sum > 0) {
                    right--;
                    while (left < right && nums[right] == nums[right + 1])
                        right--; // skip duplicated number
                } else {
                    if (sum == 0) {
                        resultSet.add(Arrays.asList(nums[i], nums[left], nums[right]));
                        right--;
                        while (left < right && nums[right] == nums[right + 1])
                            right--;
                    }
                    left++;
                    while (left < right && nums[left] == nums[left - 1])
                        left++;
                }
            }
            i++;
            while (i < nums.length - 2 && nums[i] == nums[i - 1])
                i++;
        }
        return new ArrayList<>(resultSet);
    }

}