Step through the algorithm visually — use Play or the step buttons (inspired by AlgoMaster / visualgo).
Uber interview context: 3Sum is a Medium Two Pointers problem — Shrink the search space from both ends; works on sorted arrays and palindromes.
Use the animation above to step through each move before writing code.
Pattern: Two Pointers
Read from stdin, write to stdout. Classic interview problem #15.
3Sum — Uber interview prep · Two Pointers
Classic interview problem #15.
Input (stdin)
Line 1: integers
Output (stdout)
Count of unique triplets summing to 0
Your program must read from stdin and write the answer to stdout (no extra debug text).
-1 0 1 2 -1 -4
2
| Test | Status | Details |
|---|
Ready — edit the code above and click Run or Submit.
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
class Program
{
static int[] Ria(string line = null)
{
line ??= Console.ReadLine();
if (string.IsNullOrWhiteSpace(line)) return Array.Empty<int>();
return line.Trim().Split(new[] { ' ', ',', '\t' }, StringSplitOptions.RemoveEmptyEntries)
.Select(int.Parse).ToArray();
}
static string[] Rsa()
{
int n = int.Parse(Console.ReadLine());
var arr = new string[n];
for (int i = 0; i < n; i++) arr[i] = Console.ReadLine();
return arr;
}
static void W(params object[] parts) => Console.WriteLine(string.Join(" ", parts));
static void Wb(bool v) => Console.WriteLine(v ? "true" : "false");
static void Wi(int v) => Console.WriteLine(v);
static void Ws(string v) => Console.WriteLine(v);
static void Main()
{
var nums = Ria();
Array.Sort(nums);
int count = 0;
for (int i = 0; i < nums.Length; i++) {
if (i > 0 && nums[i] == nums[i - 1]) continue;
int l = i + 1, r = nums.Length - 1;
while (l < r) {
int sum = nums[i] + nums[l] + nums[r];
if (sum == 0) { count++; l++; while (l < r && nums[l] == nums[l - 1]) l++; }
else if (sum < 0) l++; else r--;
}
}
Wi(count);
}
}Try solving on your own first, then reveal the official answer.