3 Sum

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.

• Time: O(n^2) Space: O(1)

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