015_3Sum.java

// Given an array nums of n integers, are there elements a, b, c in nums 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.
//
// Example:
//
//
// Given array nums = [-1, 0, 1, 2, -1, -4],
//
// A solution set is:
// [
//   [-1, 0, 1],
//   [-1, -1, 2]
// ]
//
//


class Solution {
    public List<List<Integer>> threeSum(int[] nums) {
    Arrays.sort(nums);
    List<List<Integer>> res = new LinkedList<>(); 
    for (int i = 0; i < nums.length-2; i++) {
        if (i == 0 || (i > 0 && nums[i] != nums[i-1])) {
            int lo = i+1, hi = nums.length-1, sum = 0 - nums[i];
            while (lo < hi) {
                if (nums[lo] + nums[hi] == sum) {
                    res.add(Arrays.asList(nums[i], nums[lo], nums[hi]));
                    while (lo < hi && nums[lo] == nums[lo+1]) lo++;
                    while (lo < hi && nums[hi] == nums[hi-1]) hi--;
                    lo++; hi--;
                } else if (nums[lo] + nums[hi] < sum) lo++;
                else hi--;
           }
        }
    }
    return res;
    }
}