MEDIUM Two Pointers
392. Is Subsequence
📖 Problem
Given two strings s and t, return true if s is a subsequence of t, or false otherwise. A subsequence of a string is a new string that is formed from the original string by deleting some (can be none) of the characters without disturbing the relative positions of the remaining characters.
🧠 Visual Learning Aid
1 Model the input into the right structure
2 Choose the core technique and invariant
3 Execute step-by-step with a sample
4 Validate complexity and edge cases
JS/TS Refreshers
- •Array methods (`push`, `pop`, `shift`, `slice`)
- •Object/Map/Set usage patterns
- •Function parameter and return typing
- •In-place array updates
- •Sorted array traversal
- •Boundary condition checks
Logical Thinking Concepts
- •Define invariants before coding
- •Check edge cases first (`[]`, single element, duplicates)
- •Estimate time/space before implementation
- •Apply Two Pointers reasoning pattern
💡 Approach
- → Use two pointers, one for each string
- → Move pointer in t when match found
- → If s pointer reaches end, check if t pointer also at end
- → Time: O(n + m), Space: O(1)
🧭 Prerequisites
🛠️ Hints & Pitfalls
Hints
- •Use two pointers, one for each string
- •Move pointer in t when match found
- •If s pointer reaches end, check if t pointer also at end
Common Pitfalls
- •Time: O(n + m), Space: O(1)
🧪 Test Cases
Hidden tests on submit: 1
Test Case 1
Not run Input:
isSubsequence('abc', 'ahbgdc'); Expected:
false Test Case 2
Not run Input:
isSubsequence('axc', 'ahbgdc'); Expected:
false Test Case 3
Not run Input:
isSubsequence('', 'abc'); Expected:
false 📝 Code Editor
📤 Output