Check If a Word Occurs As a Prefix of Any Word in a Sentence - Solution & Explanation

Problem Overview: Given a sentence containing words separated by spaces and a searchWord, return the 1-indexed position of the first word where searchWord appears as its prefix. If no word starts with that prefix, return -1.

Approach 1: Split and Compare using Simple Loop (Time: O(n * k), Space: O(n))

Split the sentence into individual words using the space delimiter, then iterate through the resulting array. For each word, check whether its first k characters match searchWord where k = searchWord.length. Most languages provide built-in helpers like startsWith or substring comparison, which keeps the implementation short and readable. This approach relies entirely on basic string operations and works well when clarity is more important than minimizing auxiliary memory.

Approach 2: Two-Pointer Traversal (Time: O(n), Space: O(1))

Instead of splitting the sentence, scan the string directly using two pointers. One pointer walks through the sentence character by character while another checks characters against searchWord when the start of a word is detected. A word boundary occurs at index 0 or immediately after a space. From each boundary, compare characters sequentially until either the prefix fully matches or a mismatch occurs. This avoids allocating an array of words and performs a single pass through the sentence. The technique is a practical use of two pointers combined with lightweight string matching.

Recommended for interviews: Both approaches are acceptable because the constraints are small and the logic is straightforward. Starting with the split-based loop demonstrates clear reasoning and correctness. The two-pointer traversal shows stronger control over string processing and space optimization, which interviewers often appreciate when discussing performance tradeoffs.