Leetcode #751: IP to CIDR
In this guide, we solve Leetcode #751 IP to CIDR in Python and focus on the core idea that makes the solution efficient.
You will see the intuition, the step-by-step method, and a clean Python implementation you can use in interviews.
Problem Statement
An IP address is a formatted 32-bit unsigned integer where each group of 8 bits is printed as a decimal number and the dot character '.' splits the groups. For example, the binary number 00001111 10001000 11111111 01101011 (spaces added for clarity) formatted as an IP address would be "15.136.255.107".
Quick Facts
- Difficulty: Medium
- Premium: Yes
- Tags: Bit Manipulation, String
Intuition
The problem structure lets us track state with bitwise operations.
Bit operations are constant time and avoid extra memory.
Approach
Apply XOR/AND/OR and shifts to maintain the required invariant.
Aggregate the result in a single pass.
Steps:
- Identify a bitwise invariant.
- Combine values with bit operations.
- Return the aggregated result.
Example
Input: ip = "255.0.0.7", n = 10
Output: ["255.0.0.7/32","255.0.0.8/29","255.0.0.16/32"]
Explanation:
The IP addresses that need to be covered are:
- 255.0.0.7 -> 11111111 00000000 00000000 00000111
- 255.0.0.8 -> 11111111 00000000 00000000 00001000
- 255.0.0.9 -> 11111111 00000000 00000000 00001001
- 255.0.0.10 -> 11111111 00000000 00000000 00001010
- 255.0.0.11 -> 11111111 00000000 00000000 00001011
- 255.0.0.12 -> 11111111 00000000 00000000 00001100
- 255.0.0.13 -> 11111111 00000000 00000000 00001101
- 255.0.0.14 -> 11111111 00000000 00000000 00001110
- 255.0.0.15 -> 11111111 00000000 00000000 00001111
- 255.0.0.16 -> 11111111 00000000 00000000 00010000
The CIDR block "255.0.0.7/32" covers the first address.
The CIDR block "255.0.0.8/29" covers the middle 8 addresses (binary format of 11111111 00000000 00000000 00001xxx).
The CIDR block "255.0.0.16/32" covers the last address.
Note that while the CIDR block "255.0.0.0/28" does cover all the addresses, it also includes addresses outside of the range, so we cannot use it.
Python Solution
Complexity
The time complexity is O(n). The space complexity is O(1).
Edge Cases and Pitfalls
Watch for boundary values, empty inputs, and duplicate values where applicable. If the problem involves ordering or constraints, confirm the invariant is preserved at every step.
Summary
This Python solution focuses on the essential structure of the problem and keeps the implementation interview-friendly while meeting the constraints.