3Sum: the idea to remember.

The core insight

Sort, fix one value, and run a two pointer search on the suffix. Skip duplicate fixed values and duplicate pairs.

Why it works

For each fixed value, sorted order lets pointer movements exhaust all feasible complementary pairs.

The mistake to avoid

Skip duplicates after recording a valid triplet; do not accidentally skip distinct answers.

Commented Java solution

Use java.util.* imports and a surrounding solution class. Tree snippets assume the standard TreeNode definition; linked lists use ListNode and Clone Graph uses a Node with val and neighbors; Copy Random List uses Node with val, next and random. Read the input assumptions above.

List<List<Integer>> threeSum(int[] nums) {
    Arrays.sort(nums);
    List<List<Integer>> out = new ArrayList<>();
    for (int i = 0; i < nums.length - 2; i++) {
        if (i > 0 && nums[i] == nums[i - 1]) continue;
        int l = i + 1, r = nums.length - 1;
        while (l < r) {
            long sum = (long) nums[i] + nums[l] + nums[r];
            if (sum < 0) l++;
            else if (sum > 0) r--;
            else {
                out.add(Arrays.asList(nums[i], nums[l], nums[r]));
                l++; r--;
                while (l < r && nums[l] == nums[l - 1]) l++;
                while (l < r && nums[r] == nums[r + 1]) r--;
            }
        }
    }
    return out;
}

Make the next connection