1768. Merge Strings Alternately 

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class Solution {
    public String mergeAlternately(String word1, String word2) {
        StringBuilder sb = new StringBuilder();

        int i = 0, j = 0;
        while (i < word1.length() || j < word2.length()) {
            if (i < word1.length()) {
                sb.append(word1.charAt(i++));
            }
            if (j < word2.length()) {
                sb.append(word2.charAt(j++));
            }
        }

        return sb.toString();
    }
}

References

1768. Merge Strings Alternately