LeetCode 917: Reverse Only Letters

Meta Data

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Difficulty: easy First Attempt: 2022-09-25

• Submissions: Wrong Answer → Runtime Error ×3 → Time Limit Exceeded ×2 → Wrong Answer ×2 → Runtime Error → Wrong Answer → Runtime Error → Wrong Answer ×9 → Runtime Error → Wrong Answer → Accepted → Wrong Answer ×2 → Accepted ×2

Approach

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The same two pointers as LC 125 (Two Pointers Template 1): move left past non-letters, move right past non-letters, and swap only when both are letters. Python strings are immutable, so convert to a list and join at the end.

My Attempts

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This took 27 submissions, the most in this batch:

• Attempt 1 started both pointers at 0 (i = j = 0), so the loop never ran, and it tried to swap characters inside a string.
• Then came many “store the letters and fill them back” variants with off-by-one errors: a single "z" returned an empty string or "zz", and "a7" lost its letter, returning "7".
• The 2026 rewrite with two pointers got it right in one go.

Solution

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class Solution:
    def reverseOnlyLetters(self, s: str) -> str:
        a = list(s)
        l, r = 0, len(a) - 1
        while l < r:
            if not a[l].isalpha():
                l += 1
            elif not a[r].isalpha():
                r -= 1
            else:
                a[l], a[r] = a[r], a[l]
                l, r = l + 1, r - 1
        return "".join(a)

Complexity

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Time O(n), space O(n).

Mistakes to Avoid

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• Strings cannot be modified in place; use a list.
• The pointers start at the head and the tail.

Interview Explanation

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I convert to a list and move two pointers that skip non-letters, swapping when both are letters, then join back. Time O(n).