Problem editorial

Merge Strings Alternately

EasyTwo PointersStringTwo PointersIteration

Merge Strings Alternately: the approach in brief

The problem

Implement mergeAlternately to combine the characters of word1 and word2 into one string. Begin with the first character of word1, then the first character of word2, and continue by taking the next character from each string in turn whenever that string still has an unused character.

Pattern

Merge Strings Alternately is a Two Pointers problem, in the In-place Array Modification subpattern. The pattern covers fast-slow, converging, expanding from center, and in-place modification techniques

Approach outline

Keep track of the next unused position in each string.
At each step, add a character from `word1` if its position is valid, then do the same for `word2`.
Continue until both positions have reached the ends of their respective strings.

Complexity

The reference solution runs in O(word1.length() + word2.length()) time and O(word1.length() + word2.length()) for the returned string; O(1) auxiliary space excluding the output space.

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Practise Merge Strings Alternately and the problems around it

Merge Strings Alternately is a Two Pointers problem, in the In-place Array Modification subpattern. Once the approach above makes sense, write it yourself and then work through the neighbouring problems that reuse it.

Merge Strings Alternately