1768. Merge Strings Alternately

1. Description

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.

2. Example

Example 1

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

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

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

3. Constraints

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

4. Solutions

Two Pointers

m = word1.size(), n = word2.size()
Time complexity: O(m + n)
Space complexity: O(1)

class Solution {
public:
    string mergeAlternately(const string &word1, const string &word2) {
        const int m = word1.size(), n = word2.size();
        string merged_string;
        merged_string.reserve(m + n);
        for (int i = 0; i < m || i < n; ++i) {
            if (i < m) {
                merged_string.push_back(word1[i]);
            }
            if (i < n) {
                merged_string.push_back(word2[i]);
            }
        }

        return merged_string;
    }
};