ThreeSum.java

/* Given an integer array nums, return all the triplets [nums[i], nums[j], nums[k]]
 * such that i != j, i != k, and j != k, and nums[i] + nums[j] + nums[k] == 0.
 * Notice that the solution set must not contain duplicate triplets.
 * 
 * Example 1 : Input : nums = [-1,0,1,2,-1,-4]
 * Output : [[-1,-1,2],[-1,0,1]]
 * Explanation :
 * nums[0] + nums[1] + nums[2] = (-1) + 0 + 1 = 0.
 * nums[1] + nums[2] + nums[4] = 0 + 1 + (-1) = 0.
 * nums[0] + nums[3] + nums[4] = (-1) + 2 + (-1) = 0.
 * The distinct triplets are [-1,0,1] and [-1,-1,2].
 * Notice that the order of the output and the order of the triplets does not matter.
 * 
 * Example 2 : Input : nums = [0,1,1]
 * Output: []
 * Explanation: The only possible triplet does not sum up to 0.
 * 
 * Example 3 : Input: nums = [0,0,0]
 * Output: [[0,0,0]]
 * Explanation: The only possible triplet sums up to 0. */


class ThreeSum {
	
    public List<List<Integer>> threeSum(int[] nums) {

        Arrays.sort(nums);

        List<List<Integer>> result = new LinkedList<>();

        for(int i=0;i<nums.length-2;i++){
            if(i==0 || (i>0 && nums[i]!=nums[i-1])){
                int left = i+1;
                int right = nums.length-1;
                int sum = 0-nums[i];
                
                while(left < right){
                    if(nums[left]+nums[right] == sum){     
                        result.add(Arrays.asList(nums[i],nums[left],nums[right]));

                        while(left < right && nums[left]==nums[left+1]){
                           left++;
                        }
                        while(left < right && nums[right] == nums[right-1]){
                           right--;
                        }

                        left++;
                        right--;
                    }
                    else if(nums[left] + nums[right] < sum){ 
                      left++;
                    }
                    else {
                       right--;
                   }
                }
            }
        } 
        return result;
    }
	

}