3Sum — LeetCode 15 Python Solution
MediumArrayTwo PointersSorting
- Problem
- #15
- Pattern
- Two Pointers
- Reading time
- 5 min
- Source
- leetcode.com
The problem
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
- 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.
Python solution
Python
class Solution:
def threeSum(self, nums: List[int]) -> List[List[int]]:
nums.sort()
n = len(nums)
ans = []
for i in range(n - 2):
if nums[i] > 0:
break
if i and nums[i] == nums[i - 1]:
continue
j, k = i + 1, n - 1
while j < k:
x = nums[i] + nums[j] + nums[k]
if x < 0:
j += 1
elif x > 0:
k -= 1
else:
ans.append([nums[i], nums[j], nums[k]])
j, k = j + 1, k - 1
while j < k and nums[j] == nums[j - 1]:
j += 1
while j < k and nums[k] == nums[k + 1]:
k -= 1
return ansComplexity
| Measure | Complexity |
|---|---|
| Time | O(n^2) |
| Space | O(\log n) auxiliary |
Pattern: Two Pointers
Use the order already in the input to discard half the search space at every step. LeetCode 15. 3Sum is filed here on both counts: the reference solution below belongs to the algorithm family this hub collects, and LeetCode tags it Two Pointers.
The two pointers guide has the Python template for the pattern and the 201 LeetCode problems that use it.
Related problems
On study lists
This problem is on Blind 75, NeetCode 150, Grind 75 and Top Interview 150.
Frequently asked questions
- How hard is LeetCode 15. 3Sum?
- LeetCode 15. 3Sum is rated Medium on LeetCode.
- What is the time complexity of LeetCode 15. 3Sum?
- The Python solution on this page runs in O(n^2).
- What is the space complexity of LeetCode 15. 3Sum?
- The Python solution on this page uses O(\log n) auxiliary space.
- What topics does LeetCode 15. 3Sum cover?
- LeetCode 15. 3Sum is tagged Array, Two Pointers and Sorting on LeetCode.