_15.java

package com.fishercoder.solutions;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

/**
 * 15. 3Sum
 *
 * 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 _15 {

    public static class Solution1 {
        public List<List<Integer>> threeSum(int[] nums) {
            Arrays.sort(nums);
            List<List<Integer>> result = new ArrayList<>();
            for (int i = 0; i < nums.length - 2; i++) {
                if (i >= 1 && nums[i] == nums[i - 1]) {
                    continue;
                }
                int left = i + 1;
                int right = nums.length - 1;
                while (left < right) {
                    int sum = nums[i] + nums[left] + nums[right];
                    if (sum == 0) {
                        result.add(Arrays.asList(nums[i], nums[left], nums[right]));

                        while (left < right && nums[left] == nums[left + 1]) {
                            left++;
                        }

                        while (right > left && nums[right] == nums[right - 1]) {
                            right--;
                        }
                        left++;
                        right--;
                    } else if (sum > 0) {
                        right--;
                    } else {
                        left++;
                    }
                }
            }
            return result;
        }
    }
}