Difficulty: Medium
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]
]
Language: Java
class Solution {
public List<List<Integer>> threeSum(int[] nums) {
if (nums.length < 3) {
return Collections.emptyList();
}
Arrays.sort(nums); // 排序用于剔除重复项
Set<List<Integer>> resultSet = new HashSet<>();
Map<Integer, List<Integer>> index = new HashMap<>();
// List<Integer> numList = new ArrayList<>();
for (int i = 0; i < nums.length; i++) {
index.computeIfAbsent(nums[i], k -> new ArrayList<>());
index.get(nums[i]).add(i);
}
for (int i = 0; i < nums.length; i++) {
if (nums[i] > 0) {
continue;
}
if (i > 0 && nums[i] == nums[i - 1]) {
continue;
}
for (int j = i + 1; j < nums.length; j++) {
int sum = nums[i] + nums[j];
if (-sum < nums[j] || -sum > nums[nums.length - 1]) {
continue;
}
List<Integer> integers = Arrays.asList(nums[i], nums[j], -sum);
if (resultSet.contains(integers)) {
continue;
}
if (index.get(-sum) != null) {
for (Integer integer : index.get(-sum)) {
if (integer > j) {
resultSet.add(integers);
}
}
}
}
}
return new ArrayList<>(resultSet);
}
}