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NeetCode #193LC-246EasyTwo Pointers
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#193 · #246 · Strobogrammatic Number(中心对称数)

📌 Problem Statement & Constraints

A strobogrammatic number is one that looks the same when rotated 180 degrees. The valid digit pairs are 0-0, 1-1, 6-9 and 8-8. Given a string num representing an integer, return whether it is strobogrammatic. Constraints: 1 <= num.length <= 50; num consists of digits only.

💡 Core Algorithmic Approaches

  1. Two pointers from the ends: each character must map to its rotation partner.
  2. The mapping is {'0': '0', '1': '1', '6': '9', '8': '8', '9': '6'}; any other digit makes the number invalid.
  3. Compare num[i] with the rotation of num[j]; on a mismatch, return false.
  4. The middle character (for odd lengths) must map to itself, which the general comparison handles.

💻 Benchmark Python3 Implementation

class Solution:
    def isStrobogrammatic(self, num: str) -> bool:
        rot = {"0": "0", "1": "1", "6": "9", "8": "8", "9": "6"}
        i, j = 0, len(num) - 1
        while i <= j:
            if num[i] not in rot or num[j] not in rot:
                return False
            if rot[num[i]] != num[j]:   # rotation partner must match
                return False
            i += 1
            j -= 1
        return True

⚡ Complexity Deep Dive

⏱️ Time Complexity
O(n): each character is examined once.
💾 Space Complexity
O(1): the mapping table is fixed.

⚠️ Interview Pitfalls & Follow-ups

  • Assuming the rotation is just a reversal: 180-degree rotation reverses the order and maps each digit, so both steps are needed.
  • Forgetting that 2, 3, 4, 5, 7 are invalid: their presence makes the answer false.
  • Using i < j as the loop condition: for odd lengths the middle character must still be checked, so the condition is i <= j.
  • Checking num[i] == rot[num[j]]: equivalent here because the mapping is an involution, but the form rot[num[i]] == num[j] reads more naturally.