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.

Difficulty Level : Easy.

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

Constraints:

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

Optimal Solution:

Time Complexity — O(N).

Space Complexity — O(N).

class Solution {
    public String mergeAlternately(String word1, String word2) {
    StringBuilder merged = new StringBuilder(); 
    int left=0;
    while(left<word1.length() || left<word2.length()){
        if(left<word1.length()) merged.append(word1.charAt(left));
        if(left<word2.length()) merged.append(word2.charAt(left));
       left++;
    }  
    return merged.toString();
    }
}

Proof:

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