3Sum

ID: 15; medium

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Solution 1 (Java)

class Solution {
    public List<List<Integer>> threeSum(int[] nums) {
        Arrays.sort(nums);
        List<List<Integer>> res = new ArrayList<>();
        for (int i = 0; i < nums.length && nums[i] <= 0; i++) {
            if (i == 0 || nums[i] != nums[i - 1]) {
                threeSumHelper(i, nums, res);
            }
        }
        return res;
    }
    
    private void threeSumHelper(int i, int[] nums, List<List<Integer>> res) {
        int low = i + 1;
        int high = nums.length - 1;
        while (low < high) {
            int sum = nums[i] + nums[low] + nums[high];
            if (sum < 0) {
                low++;
            } else if (sum > 0) {
                high--;
            } else {
                res.add(Arrays.asList(nums[i], nums[low++], nums[high--]));
                while (low < high && nums[low] == nums[low - 1]) {
                    low++;
                }
            }
        }
    }
}

// [-4, -1, -1, 0, 1, 2]