leetcode: 15. 3Sum

Difficulty

Medium

Description

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]
]

Solution

Time Complexity: O(n^2)

class Solution {
    public List<List<Integer>> threeSum(int[] nums) {
        List<List<Integer>> ret = new ArrayList<>();
        
        Arrays.sort(nums);
        for (int i = 0; i + 2 < nums.length; i++) {
            if (i > 0 && nums[i] == nums[i - 1]) continue;   //避免重复元组
            int j = i + 1;
            int k = nums.length - 1;
            int target = -nums[i];
            while (j < k) {
                if (nums[j] + nums[k] == target) {
                    ret.add(Arrays.asList(nums[i], nums[j], nums[k]));
                    j++;
                    k--;
                    while (j < k && nums[j] == nums[j - 1]) j++;  //避免重复元组
                    while (j < k && nums[k] == nums[k + 1]) k--;  //避免重复元组
                } else if (nums[j] + nums[k] > target)
                    k--;
                else
                    j++;
            }
        }
        return ret;
    }
}