Lesson 2 � Beginner-Intermediate

Two Pointer Technique

Two pointer technique arrays mein sabse zyada use hone wala pattern hai. Do pointers ek array pe kaam karte hain aur O(n�) ko O(n) mein convert karte hain. Interview mein ye technique 80% array questions mein kaam aati hai.

Two Pointer kya hai?

Two Pointer ke 2 Patterns

Two pointer technique basically do types ki hoti hai. Dono samajh lo, bahut saare questions solve ho jayenge:

# Pattern 1: Opposite Direction (Left-Right)
# Jab array sorted ho aur opposite ends se kaam karna ho

arr = [1, 2, 3, 4, 5, 6]
left = 0
right = len(arr) - 1

while left < right:
 current_sum = arr[left] + arr[right]
 if current_sum == 7:
 print(f"Found: {arr[left]} + {arr[right]} = 7")
 break
 elif current_sum < 7:
 left += 1 # Sum badhana hai
 else:
 right -= 1 # Sum kam karna hai

# Pattern 2: Same Direction (Slow-Fast)
# Jab cycle detect karna ho ya duplicate remove karna ho

arr = [1, 1, 2, 2, 3, 4, 4, 5]
slow = 0 # Slow pointer � unique elements ki jagah

for fast in range(1, len(arr)):
 if arr[fast] != arr[slow]:
 slow += 1
 arr[slow] = arr[fast]

# arr mein first (slow+1) elements unique hain
print(arr[:slow+1]) # [1, 2, 3, 4, 5]

Step-by-Step Example: Two Sum in Sorted Array

Maan lo tumhe sorted array mein do numbers find karne hain jinka sum target ke barabar ho. Brute force approach O(n�) hogi � har element ke saath sab elements check karna. Two pointer se O(n) ho jayega:

def two_sum_sorted(arr, target):
 left = 0
 right = len(arr) - 1
 
 while left < right:
 current_sum = arr[left] + arr[right]
 
 if current_sum == target:
 return [left, right] # Mil gaya!
 elif current_sum < target:
 left += 1 # Sum chhota hai, left badhao
 else:
 right -= 1 # Sum bada hai, right ghatao
 
 return [-1, -1] # Nahi mila

# Example
arr = [1, 2, 3, 4, 6, 8, 10]
print(two_sum_sorted(arr, 10)) # [1, 4] ✓ arr[1]+arr[4] = 2+8 = 10

Visualize karo � left pointer pe 1 hai, right pe 10. Sum = 11, target se zyada hai. Right ko move karo. Ab left=1 (value 2), right=6 (value 8). Sum = 10, mil gaya! Ye brute force se bahut fast hai.

Palindrome Check with Two Pointers

Ek string palindrome hai ya nahi � ye check karna two pointer se bahut easy hai. Dono taraf se compare karte jao:

def is_palindrome(s):
 left = 0
 right = len(s) - 1
 
 while left < right:
 if s[left] != s[right]:
 return False # Mismatch mila
 left += 1
 right -= 1
 
 return True # Sab match hai

print(is_palindrome("racecar")) # True
print(is_palindrome("hello")) # False

Two Pointer Variations

Try it: code khud likho

Exercise

Common mistakes

• Pointers ko correctly move nahi karna: Agar sum chhota hai toh left badhana hai, agar bada hai toh right ghatana hai. Ulta karoge toh infinite loop mein fas jaoge.
• Infinite loop: while left < right ki jagah while left <= right kabhi mat likho jab sum check kar rahe ho � same element twice count ho jayega.
• Sorted array bhool jaana: Two pointer technique sorted arrays pe kaam karti hai. Agar array sorted nahi hai toh pehle sort karo ya brute force use karo.
• Edge cases skip karna: Empty array, single element, ya sab elements same � ye sab test karo. Interview mein ye galat hota hai.