ภาพรวม
ใช้ HashSet<T> และ LINQ Intersect หรือ Any เพื่อตรวจสอบว่าสอง Collection มีสมาชิกร่วมกันหรือไม่
โค้ดตัวอย่าง
using System;
using System.Collections.Generic;
using System.Linq;
public class Program {
public static void Main() {
int[] arr1 = { 65, 57, 63, 98 };
int[] arr2 = { 43, 65, 33, 57 };
Console.WriteLine($"Array 1: [{string.Join(", ", arr1)}]");
Console.WriteLine($"Array 2: [{string.Join(", ", arr2)}]");
// วิธีที่ 1: HashSet + Any (เร็วที่สุด O(n+m))
var set2 = new HashSet<int>(arr2);
bool hasCommon = arr1.Any(x => set2.Contains(x));
Console.WriteLine($"Has common elements: {hasCommon}"); // True
// วิธีที่ 2: LINQ Intersect (สร้าง collection ใหม่)
var common = arr1.Intersect(arr2).ToArray();
Console.WriteLine($"Common elements: [{string.Join(", ", common)}]"); // 65, 57
// วิธีที่ 3: Manual Two-pointer (ต้อง Sorted ก่อน)
Array.Sort(arr1);
Array.Sort(arr2);
int i = 0, j = 0;
var commonManual = new List<int>();
while (i < arr1.Length && j < arr2.Length) {
if (arr1[i] == arr2[j]) {
commonManual.Add(arr1[i]);
i++; j++;
} else if (arr1[i] < arr2[j]) {
i++;
} else {
j++;
}
}
Console.WriteLine($"Common (manual): [{string.Join(", ", commonManual)}]");
}
}
ผลลัพธ์
Array 1: [65, 57, 63, 98]
Array 2: [43, 65, 33, 57]
Has common elements: True
Common elements: [65, 57]
Common (manual): [57, 65]
Performance Comparison
| Method | Time Complexity | Space | Returns |
|---|---|---|---|
| HashSet + Any | O(n + m) | O(m) | bool (มีร่วมหรือไม่) |
| LINQ Intersect | O(n + m) | O(min(n,m)) | Common elements |
| Two Pointer (Sorted) | O(n log n + m log m) | O(1) extra | Common elements (sorted) |
| Nested Loop | O(n × m) | O(1) | Common elements |
Extension Method สำหรับ Reusability
public static class CollectionExtensions {
public static bool HasCommonElements<T>(
this IEnumerable<T> first,
IEnumerable<T> second,
IEqualityComparer<T> comparer = null) {
var set = new HashSet<T>(second, comparer);
return first.Any(set.Contains);
}
public static IEnumerable<T> GetCommonElements<T>(
this IEnumerable<T> first,
IEnumerable<T> second) {
var set = new HashSet<T>(second);
return first.Where(set.Contains);
}
}
// ใช้งาน
bool hasCommon = arr1.HasCommonElements(arr2);
var common = arr1.GetCommonElements(arr2).ToList();
