Two Pointers

INFO

A technique that uses two indices to traverse a data structure—often in opposite directions or at different speeds—to solve problems more efficiently. It’s commonly used in arrays, strings, and linked lists to reduce time complexity from O(n²) to O(n).

Two Pointers

Properties

• Uses two pointers (e.g., left and right, or slow and fast)
• Often applied to sorted arrays, subarray problems, and linked list traversal
• Can be used for window-based problems, cycle detection, and partitioning
• Reduces nested loops into linear scans

Common Patterns

• Converging Pointers: Start at both ends and move toward the center
  • Used in palindrome checks, pair sums, and partitioning
• Sliding Window: Move both pointers forward to maintain a dynamic range
  • Used in substring problems, max/min subarrays
• Fast–Slow Pointers: One pointer moves faster than the other
  • Used in cycle detection, finding middle of linked list

Common Operations

• Pair Sum: Find two numbers that add up to a target (left + right)
• Reverse: Swap elements from both ends (arr[left], arr[right] = arr[right], arr[left])
• Window Expansion/Contraction: Adjust pointers based on constraints
• Cycle Detection: Use fast and slow pointers to detect loops in linked lists