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]
]

Pay attention to avoid duplicates:

if nums[i] == nums[i-1], which means if there is a triplet [i-1, l, r] == 0, you need to skip i, same applies to l, l-1 and r, r+1

public class Solution {
    public List<List<Integer>> threeSum(int[] nums) {
        List<List<Integer>> res = new ArrayList<>();
        if(nums == null || nums.length == 0) return res;

        Arrays.sort(nums);
        for(int i=0; i< nums.length -2; i++){
            if( i > 0 && nums[i] == nums[i-1]) continue;

            int l = i+1;
            int r = nums.length-1;
            while(l < r){
                int sum = nums[i] + nums[l] + nums[r];
                if(sum ==0){
                    List<Integer> t = new ArrayList<>();
                    t.add(nums[i]);
                    t.add(nums[l]);
                    t.add(nums[r]);
                    res.add(t);
                    l++;
                    r--;
                    while(l < r && nums[l] == nums[l-1]) l++;
                    while(l < r && nums[r] == nums[r+1]) r--;
                }else if(sum < 0){
                    l++;
                }else{
                    r--;
                }
            }
        }
        return res;
    }
}