How hard is LeetCode 2843. Count Symmetric Integers?

LeetCode 2843. Count Symmetric Integers is rated Easy on LeetCode.

What is the time complexity of LeetCode 2843. Count Symmetric Integers?

The Python solution on this page runs in O(n \times \log m).

What is the space complexity of LeetCode 2843. Count Symmetric Integers?

The Python solution on this page uses O(\log m) auxiliary space.

What topics does LeetCode 2843. Count Symmetric Integers cover?

LeetCode 2843. Count Symmetric Integers is tagged Math and Enumeration on LeetCode.

Count Symmetric Integers — LeetCode 2843 Python Solution

EasyMathEnumeration

Problem: #2843
Pattern: Math and Number Theory
Reading time: 2 min
Source: leetcode.com

The problem

You are given two positive integers low and high. An integer x consisting of 2 * n digits is symmetric if the sum of the first n digits of x is equal to the sum of the last n digits of x.

Example

Input: low = 1, high = 100
Output: 9
Explanation: There are 9 symmetric integers between 1 and 100: 11, 22, 33, 44, 55, 66, 77, 88, and 99.

Python solution

Python

class Solution:
    def countSymmetricIntegers(self, low: int, high: int) -> int:
        def f(x: int) -> bool:
            s = str(x)
            if len(s) & 1:
                return False
            n = len(s) // 2
            return sum(map(int, s[:n])) == sum(map(int, s[n:]))

        return sum(f(x) for x in range(low, high + 1))

Complexity

Measure	Complexity
Time	O(n \times \log m)
Space	O(\log m) auxiliary

Pattern: Math and Number Theory

Find the closed form, the invariant, or the modular identity — and skip the loop entirely. LeetCode 2843. Count Symmetric Integers is filed here on both counts: the reference solution below belongs to the algorithm family this hub collects, and LeetCode tags it Math.

The math and number theory guide has the Python template for the pattern and the 485 LeetCode problems that use it.

Frequently asked questions

How hard is LeetCode 2843. Count Symmetric Integers?: LeetCode 2843. Count Symmetric Integers is rated Easy on LeetCode.
What is the time complexity of LeetCode 2843. Count Symmetric Integers?: The Python solution on this page runs in O(n \times \log m).
What is the space complexity of LeetCode 2843. Count Symmetric Integers?: The Python solution on this page uses O(\log m) auxiliary space.
What topics does LeetCode 2843. Count Symmetric Integers cover?: LeetCode 2843. Count Symmetric Integers is tagged Math and Enumeration on LeetCode.