Problem statement

You are given two strings word1 and word2. Merge them by adding letters in alternating order, starting with word1.

If one string is longer, append its remaining letters to the end of the merged string.

Example 1

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

Example 2

Input: word1 = "ab", word2 = "pqrs"
Output: apbqrs
Explanation: After "ab" runs out, the rest of word2 is appended.

Example 3

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

Constraints

• 1 <= word1.length, word2.length <= 30
• Both consist of lowercase English letters.

Hints for Merge Strings Alternately

1. Hint 1

   Loop up to the length of the longer string.

2. Hint 2

   At each step append word1[i] if it exists, then word2[i] if it exists.

3. Hint 3

   That single loop handles the leftover tail without a separate append step.

How to solve Merge Strings Alternately

Loop to the longer length, appending word1[i] then word2[i] whenever each exists; the tail of the longer word falls out naturally.

Approach

1. For i in 0 .. max(len1, len2) - 1: append word1[i] if i < len1, then word2[i] if i < len2.
2. Join.

Why it works

Alternation is index-by-index interleaving, and the bounds checks make the leftover characters of the longer string append in order once the shorter one runs out.

Complexity

• Time — O(n + m)
• Space — O(n + m)

Pitfalls

• Looping to the shorter length and appending the remainder separately also works — just remember both possible tails.

Reference solution

Python

def mergeAlternately(word1: str, word2: str) -> str:
    out = []
    for i in range(max(len(word1), len(word2))):
        if i < len(word1):
            out.append(word1[i])
        if i < len(word2):
            out.append(word2[i])
    return "".join(out)

Java

public static String mergeAlternately(String word1, String word2) {
    StringBuilder out = new StringBuilder();
    int n = Math.max(word1.length(), word2.length());
    for (int i = 0; i < n; i++) {
        if (i < word1.length()) out.append(word1.charAt(i));
        if (i < word2.length()) out.append(word2.charAt(i));
    }
    return out.toString();
}

C++

string mergeAlternately(string word1, string word2) {
    string out = "";
    size_t n = max(word1.size(), word2.size());
    for (size_t i = 0; i < n; i++) {
        if (i < word1.size()) out += word1[i];
        if (i < word2.size()) out += word2[i];
    }
    return out;
}

JavaScript

var mergeAlternately = function(word1, word2) {
    let out = "";
    const n = Math.max(word1.length, word2.length);
    for (let i = 0; i < n; i++) {
        if (i < word1.length) out += word1[i];
        if (i < word2.length) out += word2[i];
    }
    return out;
};

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