Merge Strings Alternately

Strings / Merge

Easy

Given two strings word1 and word2, merge them by alternating characters starting with word1. If one string is longer, append the remaining characters at the end.

Example


Example 1

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

Output: "apbqcr"



Example 2

Input: word1 = "ab", word2 = "pqrs"

Output: "apbqrs"



Example 3

Input: word1 = "abcd", word2 = "pq"

Output: "apbqcd"



Example 4

Input: word1 = "", word2 = "xyz"

Output: "xyz"



Example 5

Input: word1 = "x", word2 = ""

Output: "x"

Explanation

We start with two input strings called word1 and word2.
Create an empty StringBuilder named result. Think of it as a flexible box where we drop letters.
Set two counters: i = 0 for word1 and j = 0 for word2. These point to the next letter to take.
Repeat these steps while there are letters left in either word (i < word1.length() or j < word2.length()):
- If i < word1.length(), take word1.charAt(i), append it to result, then increase i by 1.
- If j < word2.length(), take word2.charAt(j), append it to result, then increase j by 1.
If one word runs out first, simply skip it and keep taking letters from the other word until it is finished.
When the loop ends, convert result to a regular string with result.toString(). That is the merged answer.

Quick example: word1 = "ab", word2 = "pqrs" → result builds as "a" → "ap" → "apb" → "apbq" → "apbqr" → "apbqrs".

Idea Map

Start

Initialize i = 0, j = 0, n1 = len(word1), n2 = len(word2), sb = new StringBuilder()

Loop while i < n1 || j < n2

True

1. If i < n1, append word1.charAt(i++)
2. If j < n2, append word2.charAt(j++)

Return sb.toString()

End

Code (Java)


class Solution {
Defines the Solution class as expected by the LeetCode environment.
    public String mergeAlternately(String word1, String word2) {
Method signature: takes two input strings and returns their alternating merge.
        StringBuilder result = new StringBuilder();
Use StringBuilder for efficient, mutable string construction.
        int length1 = word1.length();
Cache lengths to avoid repeated method calls in the loop.
        int length2 = word2.length();
Length of the second string.
        int i = 0, j = 0;
Two indices to track positions in word1 and word2.
        while (i < length1 || j < length2) {
Continue while either string has characters remaining.
            if (i < length1)
Append from word1 if there is a character available.
                result.append(word1.charAt(i++));
Append one character from word1 and increment i.
            if (j < length2)
Append from word2 if there is a character available.
                result.append(word2.charAt(j++));
Append one character from word2 and increment j.
        }
End of the loop.
        return result.toString();
Convert the StringBuilder to an immutable string and return.
    }
End of method.
}
End of class.

What is String Concatenation in Java?

String concatenation is the process of joining strings together. In Java, you can concatenate using the + operator or String.concat. Because String is immutable, repeated concatenation in loops can be inefficient — prefer StringBuilder (or StringBuffer in multi-threaded contexts) for better performance.

// Examples
String a = "Hello" + ", " + "World"; // simple
String b = "Hi".concat(" ").concat("there");
StringBuilder sb = new StringBuilder();
sb.append("A").append("B");
String c = sb.toString();

Learn more about Java String Concatenation →

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Disclaimer: This problem is for educational purposes. We encourage you to try solving the 'Merge Strings Alternately' question on LeetCode.

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