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]
]

Solution:

Tips:

sort array first, fix one number in either of the end, then from two sides to middle.

Java Code:

public class Solution {
    public List<List<Integer>> threeSum(int[] nums) {
        Arrays.sort(nums);
        List<List<Integer>> result = new ArrayList<>();
        
        for (int i = nums.length - 1; i >= 2; i--) {
            int left = 0;
            int right = i - 1;

            while (left < right) {
                int sum = -(nums[left] + nums[right]);
                if (sum == nums[i]) {
                    result.add(Arrays.asList(nums[left], nums[right], nums[i]));
                    while (++left < right && nums[left - 1] == nums[left]);
                    while (--right > left && nums[right] == nums[right + 1]);
                } else if (sum > nums[i]) {
                    left++;
                } else {
                    right--;
                }
            }

            while (i >= 2 && nums[i] == nums[i - 1]) {
                i--;
            }
        }

        return result;        
    }
}