Merge Strings Alternately

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Merge two strings by adding letters in alternating order, starting with word1. If one string is longer, append the remaining letters to the end.

Example 1
Input: word1="abc", word2="pqr" → Output: "apbqcr"
Example 2
Input: word1="ab", word2="pqrs" → Output: "apbqrs"

Solution Approach

Use two pointers advancing through both strings simultaneously. Append from word1 then word2 in turns.

  • Use pointers i and j starting at 0.
  • While either i < word1.length or j < word2.length:
  • Append word1[i] if i is valid; append word2[j] if j is valid.
  • Advance both.
Java
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();
    }
}
  • Time Complexity: O(N+M)
  • Space Complexity: O(N+M)