1768. Merge Strings Alternately

[#two-pointers]

## description

## 1768. Merge Strings Alternately

You are given two strings word1 and word2. Merge the strings by adding letters in alternating order, starting with word1. If a string is longer than the other, append the additional letters onto the end of the merged string.

Return the merged string.

Example 1:

plain text

Input: word1 = "abc", word2 = "pqr"
Output: "apbqcr"
Explanation: The merged string will be merged as so:
word1:  a   b   c
word2:    p   q   r
merged: a p b q c r

Example 2:

plain text

Input: word1 = "ab", word2 = "pqrs"
Output: "apbqrs"
Explanation: Notice that as word2 is longer, "rs" is appended to the end.
word1:  a   b 
word2:    p   q   r   s
merged: a p b q   r   s

Example 3:

plain text

Input: word1 = "abcd", word2 = "pq"
Output: "apbqcd"
Explanation: Notice that as word1 is longer, "cd" is appended to the end.
word1:  a   b   c   d
word2:    p   q 
merged: a p b q c   d

–
1 <= word1.length, word2.length <= 100
–
word1 and word2 consist of lowercase English letters.

Constraints:

## notes

### Intuition

The merge pattern alternates between strings: first character from word1, then word2, then word1, and so on. When one string is exhausted, append the remainder of the other.

### Implementation

Use two pointers (i, j) for each word and a counter k to track whose turn it is. While both pointers are in bounds, add from word1 when k is even, from word2 when odd, incrementing the respective pointer. After the main loop, append any remaining characters from whichever string isn't exhausted.

### Edge-cases

Strings of unequal length are handled by the trailing loops that append leftover characters. Empty strings work naturally—the main loop won't execute.

–
Time: O(n + m) — each character processed once
–
Space: O(1) — excluding the output string

### Complexity

## solution

python

from typing import List

class Solution:
    def mergeAlternately(self, word1: str, word2: str) -> str:
        i = j = k = 0

        res = ""
        while i < len(word1) and j < len(word2):
            if k % 2 == 0:
                res += word1[i]
                i += 1
            else:
                res += word2[j]
                j += 1
            k += 1

        while i < len(word1):
            res += word1[i]
            i += 1

        while j < len(word2):
            res += word2[j]
            j += 1

        return res


if __name__ == "__main__":
    # Include one-off tests here or debugging logic that can be run by running this file
    # e.g. print(solution.two_sum([1, 2, 3, 4], 3))
    solution = Solution()

--EOF--