Merging Strings Alternately in Python: A Two-Pointer Approach

Feb 122 min read

String manipulation is a common problem in programming, often requiring efficient solutions. One such problem is merging two strings by alternating their characters while appending any remaining characters from the longer string at the end. In this blog, we will explore an optimal approach using Python.

Problem Statement

We are given two strings, word1 and word2. Our task is to merge them by adding letters in an alternating order, starting with word1. If one string is longer than the other, the additional characters should be appended at the end of the merged string.

Efficient Approach Using Two Pointers

Instead of iterating through both strings separately and concatenating characters (which is inefficient), we can leverage a two-pointer approach to efficiently merge characters from both strings.

Python Implementation:

class Solution:
    def mergeAlternately(self, word1: str, word2: str) -> str:
        merged_word = []
        len_1, len_2 = len(word1), len(word2)
        min_len = min(len_1, len_2)

        # Merge characters alternatively
        for i in range(min_len):
            merged_word.append(word1[i])
            merged_word.append(word2[i])

        # Append the remaining characters from the longer word
        merged_word.append(word1[min_len:] + word2[min_len:])

        return "".join(merged_word)

Explanation of the Code

Initialize an empty list (merged_word): Instead of concatenating strings (which is inefficient in Python), we use a list to store the merged characters.
Find the minimum length (min_len): This determines how many alternating characters can be merged before one string runs out.
Loop through the characters up to min_len: Using a for loop, we append alternating characters from word1 and word2.
Handle the remaining characters: If one string is longer than the other, the remaining characters are appended to the merged list.
Return the final string: We use "".join(merged_word) to efficiently convert the list into a string.

Time Complexity Analysis

The loop runs for min(len_1, len_2) iterations.
Appending remaining characters takes O(N) in the worst case.
Overall, the time complexity is O(N), where N is the total number of characters in both strings.

Example Runs

sol = Solution()
print(sol.mergeAlternately("abc", "pqrst"))  # Output: "apbqcrst"
print(sol.mergeAlternately("hello", "world"))  # Output: "hweolrllod"
print(sol.mergeAlternately("a", "bcd"))  # Output: "abcd"

Key Takeaways

The two-pointer approach ensures an optimal and readable solution.
Using lists (append()) instead of direct string concatenation improves efficiency.
The function gracefully handles strings of different lengths.

This method provides a simple yet effective way to merge two strings alternately, making it a great technique for string manipulation problems in competitive programming and interviews.

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