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NeetCode #189LC-557EasyTwo Pointers
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#189 · #557 · Reverse Words in a String III(反转字符串中的单词 III)

📌 Problem Statement & Constraints

Given a string s, reverse the order of characters in each word while keeping the word order and the single spaces between words. Constraints: 1 <= s.length <= 5 * 10^4; s contains printable ASCII characters without leading or trailing spaces and with single spaces between words.

💡 Core Algorithmic Approaches

  1. Split on spaces, reverse each word, and rejoin.
  2. A more memory-conscious version walks the string, finds each word's boundaries, and writes its characters in reverse into a result buffer.
  3. The split-and-join version is O(n) and far clearer; only reach for the in-place variant if the input is a mutable character array.
  4. Note that this is different from reversing the whole string or reversing the word order.

💻 Benchmark Python3 Implementation

class Solution:
    def reverseWords(self, s: str) -> str:
        return " ".join(w[::-1] for w in s.split(" "))


# In-place variant for a mutable character array
class Solution2:
    def reverseWords(self, s: List[str]) -> None:
        n = len(s)
        start = 0
        for i in range(n + 1):         # n acts as the end sentinel
            if i == n or s[i] == " ":
                lo, hi = start, i - 1
                while lo < hi:
                    s[lo], s[hi] = s[hi], s[lo]
                    lo += 1
                    hi -= 1
                start = i + 1

⚡ Complexity Deep Dive

⏱️ Time Complexity
O(n): each character is touched once (the split version additionally allocates the word list).
💾 Space Complexity
O(n) for the result; O(1) extra for the in-place variant.

⚠️ Interview Pitfalls & Follow-ups

  • Reversing the word order as well: the problem only reverses the characters within each word.
  • Splitting on whitespace with s.split(): that collapses runs of spaces; here single spaces are guaranteed, so either form works, but split(" ") is the literal reading.
  • Forgetting the end sentinel in the in-place version: the final word would never be reversed.
  • Trying to reverse in place when s is an immutable string: strings cannot be mutated; a list of characters or a rebuilt string is required.