leetcode : 3 sum

Posted 2020-08-31 notesbuddy

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.

Note: The solution set must not contain duplicate triplets.

For example, given array S = [-1, 0, 1, 2, -1, -4],

A solution set is:
[
  [-1, 0, 1],
  [-1, -1, 2]
]


tag : two pointers

public class Solution {
    public List<List<Integer>> threeSum(int[] nums) {
        List<List<Integer>> result = new ArrayList<List<Integer>>();
		if(nums == null || nums.length < 3){
			return result;
		}
		
		Arrays.sort(nums);
		
		for(int i = 0 ; i < nums.length - 2; i++){
			int a = nums[i];
			if(i != 0 && nums[i-1] == nums[i]){
				continue;
			}
			
			int left = i + 1;
			int right = nums.length - 1;
			
			while(left < right){
				int b = nums[left];
				int c = nums[right];
				int sum = a + b + c;
				if(sum == 0){
					List<Integer> tmp = new ArrayList<Integer>();
					tmp.add(a);
					tmp.add(b);
					tmp.add(c);
					result.add(tmp);
					left++;
					right--;
					while(left < right && nums[left] == nums[left-1]){
						left ++;
					}
					while(left < right && nums[right + 1] == nums[right]){
						right --;
					}
				}else if(a + b + c < 0){
					left ++;
				}else{
					right --;
				}
			}
		}
		return result;
    }
}