ThreeSumSolution.java

/**
 * Given an array S of n integers, are there elements a, b, c in S such that a + b + c = 0? Find all unique triplets in the array which gives the sum of zero.
 *
 * Note: The solution set must not contain duplicate triplets.
 *
 * For example, given array S = [-1, 0, 1, 2, -1, -4],
 *
 * A solution set is:
 * [
 * [-1, 0, 1],
 * [-1, -1, 2]
 * ]
 */
public class ThreeSumSolution {
    public List<List<Integer>> threeSum(int[] nums) {
        List<List<Integer>> result = new ArrayList<>();
        Arrays.sort(nums);
        for (int i = 0; i < nums.length - 2; i++) {
            if (i == 0 || nums[i] != nums[i - 1]) {
                int j = i + 1;
                int k = nums.length - 1;
                int target = -nums[i];
                while (j < k) {
                    if (nums[j] + nums[k] == target) {
                        result.add(Arrays.asList(nums[i], nums[j], nums[k]));
                        j++;
                        k--;
                        while (j < k && nums[j] == nums[j - 1]) j++;
                        while (j < k && nums[k] == nums[k + 1]) k--;
                    } else if (nums[j] + nums[k] < target) {
                        j++;
                    } else {
                        k--;
                    }
                }
            }
        }
        return result;
    }
}