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NeetCode #74LC-2490EasyArrays & Hashing
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#74 · #2490 · Circular Sentence(循环句子)

📌 Problem Statement & Constraints

A sentence is circular if the last character of each word equals the first character of the next word, including the wrap-around from the last word back to the first. Given a string sentence, return whether it is circular. Constraints: 1 <= sentence.length <= 500; words are separated by single spaces.

💡 Core Algorithmic Approaches

  1. Split the sentence into words.
  2. For every adjacent pair, check that the last character of the first word equals the first character of the second.
  3. Finally check the wrap-around pair: the last character of the final word must equal the first character of the first word.
  4. A single-word sentence is circular by definition, since the wrap-around pair is the word with itself.

💻 Benchmark Python3 Implementation

class Solution:
    def isCircularSentence(self, sentence: str) -> bool:
        words = sentence.split()
        for i in range(len(words)):
            a, b = words[i], words[(i + 1) % len(words)]
            if a[-1] != b[0]:          # includes the wrap-around pair
                return False
        return True

⚡ Complexity Deep Dive

⏱️ Time Complexity
O(n): splitting is linear in the sentence length and the pairwise checks are O(number of words).
💾 Space Complexity
O(n) for the word list.

⚠️ Interview Pitfalls & Follow-ups

  • Only checking adjacent pairs without the wrap-around: the last-to-first pair is explicitly required.
  • Assuming a single-word sentence is not circular: with the modulo index, the word is compared with itself, which passes.
  • Splitting on ' ': with explicit ' ' the words are fine here (single spaces guaranteed), but split() is safer against edge cases.