Introduction

The Merge Strings Alternately problem involves combining two strings by taking characters alternately from each string.

Given two strings:

  • word1
  • word2

the task is to:

  • merge characters alternately
  • append remaining characters after one string ends

This problem helps in understanding:

  • Two Pointer Technique
  • string traversal
  • alternate merging
  • conditional processing

Example

Input:word1 = "abc"
word2 = "pqr"

Output:
"apbqcr"

Explanation:
Characters are taken alternately
from both strings.


Input:
word1 = "hello"
word2 = "xy"

Output:
"hxeyllo"

Explanation:
After second string ends,
remaining characters of first string
are appended.

Constraints

1 <= word1.length,word2.length <= 100
word1 and word2
contain lowercase English letters

Approach 1 : Brute Force

Explanation

The simplest way to solve this problem is:

  1. Traverse both strings simultaneously
  2. Append characters alternately
  3. Append remaining characters

This approach directly follows the problem statement.

Steps

  1. Initialize empty result string.
  2. Traverse both strings.
  3. Append:
    • one character from word1
    • one character from word2
  4. Append remaining characters.
  5. Return merged string.

Dry Run

Input:word1 = "abc"
word2 = "pqr"
Take a and p:
"ap"

Take b and q:
"apbq"

Take c and r:
"apbqcr"

Final Result:
"apbqcr"

Brute Force Code

Complexity Analysis

Time Complexity: O(n + m)Explanation:
Both strings are traversed once.

Space Complexity: O(n + m)
Explanation:
Extra result string is created.

Approach 2 : Optimized Solution (Two Pointer Technique)

Explanation

The optimized solution uses two pointers.

The idea is:

  1. Use:
    • pointer i for word1
    • pointer j for word2
  2. Append characters alternately.
  3. Continue until both strings finish.

This provides clean traversal logic.

Steps

  1. Initialize:
    • i = 0
    • j = 0
  2. Traverse while characters remain.
  3. Append characters alternately.
  4. Move pointers.
  5. Return merged string.

Dry Run

Input:word1 = "hello"
word2 = "xy"

Initially:
i = 0
j = 0

Take h and x:
"hx"

Take e and y:
"hxey"
Second string finished

Append remaining:
"llo"

Final Result:
"hxeyllo"

Optimized Code

Complexity Analysis

Time Complexity: O(n + m)Explanation:
Each character is processed once.

Space Complexity: O(n + m)
Explanation:
Extra merged string is created.


Edge Cases
  1. One string is empty
  2. Both strings have same length
  3. Strings contain one character
  4. One string is longer
  5. Strings contain special characters

Why This Problem is Important

This problem helps in understanding:

  1. Two Pointer Technique
  2. Alternate traversal
  3. String merging
  4. Conditional processing
  5. Sequential traversal

It is one of the most common string two-pointer interview problems.

Real-World Applications

Alternate merging concepts are used in:

  1. Data streaming
  2. File merging systems
  3. Text formatting
  4. Compiler design
  5. Parallel processing systems

Common Mistakes

  1. Forgetting remaining characters
  2. Incorrect pointer updates
  3. Wrong loop condition
  4. String index errors

Interview Tips

Interviewers often expect:

  1. Two Pointer optimization
  2. Proper traversal explanation
  3. Remaining-character handling

Always explain how leftover characters are appended after one string ends.

Related Questions

  1. Merge Sorted Arrays
  2. Reverse String
  3. Reverse Vowels
  4. Valid Palindrome II
  5. String Compression

Final Takeaway

The Merge Strings Alternately problem is a fundamental two-pointer string problem that teaches alternate traversal and sequential merging techniques. Understanding this problem builds a strong foundation for advanced string manipulation and traversal interview problems.