Merge Strings Alternately

Merge Strings Alternately - Problem

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.

Input & Output

Example 1 — Basic Equal Length

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

› Output: "apbqcr"

💡 Note: Merge alternately: a from word1, p from word2, b from word1, q from word2, c from word1, r from word2

Example 2 — Different Lengths

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

› Output: "apbqrs"

💡 Note: Merge alternately: a, p, b, q, then append remaining characters from word2: r, s

Example 3 — First String Longer

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

› Output: "apbqcd"

💡 Note: Merge alternately: a, p, b, q, then append remaining characters from word1: c, d

Constraints

1 ≤ word1.length, word2.length ≤ 100
word1 and word2 consist of lowercase English letters

Visualization

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Asked in

A Amazon 15 M Microsoft 12 G Google 8

Brief HTML summary: The key insight is to use two pointers to traverse both strings simultaneously, alternating character selection. Best approach is two pointers for cleaner logic. Time: O(n + m), Space: O(n + m)

Common Approaches

✓ Brute Force Character by Character

⏱️ Time: O(n + m) Space: O(n + m)

Use a while loop to iterate through both strings simultaneously, adding characters alternately. Check bounds for each string on every iteration to handle different lengths.

Two Pointers Optimized

⏱️ Time: O(n + m) Space: O(n + m)

Maintain separate pointers for each string and alternate between them. Process the common length first, then append remaining characters from the longer string.

Brute Force Character by Character — Algorithm Steps

Step 1: Initialize result string and index counter
Step 2: Loop while either string has remaining characters
Step 3: Add character from word1 if available, then from word2 if available

Visualization

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Step-by-Step Walkthrough

Initialize

Start with empty result and index 0

Alternate

Add from word1 if available, then word2 if available

Continue

Repeat until both strings exhausted

Code -

solution.c — C

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

char* solution(char* word1, char* word2) {
    int len1 = strlen(word1);
    int len2 = strlen(word2);
    char* result = malloc((len1 + len2 + 1) * sizeof(char));
    int i = 0, pos = 0;
    
    while (i < len1 || i < len2) {
        if (i < len1) {
            result[pos++] = word1[i];
        }
        if (i < len2) {
            result[pos++] = word2[i];
        }
        i++;
    }
    
    result[pos] = '\0';
    return result;
}

int main() {
    char word1[1001], word2[1001];
    
    fgets(word1, sizeof(word1), stdin);
    word1[strcspn(word1, "\n")] = 0;
    
    fgets(word2, sizeof(word2), stdin);
    word2[strcspn(word2, "\n")] = 0;
    
    char* result = solution(word1, word2);
    printf("%s\n", result);
    free(result);
    
    return 0;
}

Time & Space Complexity

Time Complexity

⏱️

O(n + m)

Single pass through both strings where n and m are lengths of word1 and word2

✓ Linear Growth

Space Complexity

O(n + m)

Result string stores all characters from both input strings

⚡ Linearithmic Space

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Medium Frequency

~8 min Avg. Time

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Ln 1, Col 1

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Input & Output

Constraints

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Related Problems

Common Approaches

Brute Force Character by Character — Algorithm Steps

Visualization

Code -

Time & Space Complexity

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