Leetcode #2844: Minimum Operations to Make a Special Number
In this guide, we solve Leetcode #2844 Minimum Operations to Make a Special Number 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
You are given a 0-indexed string num representing a non-negative integer. In one operation, you can pick any digit of num and delete it.
Quick Facts
- Difficulty: Medium
- Premium: No
- Tags: Greedy, Math, String, Enumeration
Intuition
A locally optimal choice leads to a globally optimal result for this structure.
That means we can commit to decisions as we scan without backtracking.
Approach
Sort or preprocess if needed, then repeatedly take the best available local choice.
Maintain the minimal state necessary to validate the greedy decision.
Steps:
- Sort or preprocess as needed.
- Iterate and pick the best local option.
- Track the current solution.
Example
Input: num = "2245047"
Output: 2
Explanation: Delete digits num[5] and num[6]. The resulting number is "22450" which is special since it is divisible by 25.
It can be shown that 2 is the minimum number of operations required to get a special number.
Python Solution
class Solution:
def minimumOperations(self, num: str) -> int:
def dfs(i: int, k: int) -> int:
if i == n:
return 0 if k == 0 else n
ans = dfs(i + 1, k) + 1
ans = min(ans, dfs(i + 1, (k * 10 + int(num[i])) % 25))
return ans
n = len(num)
return dfs(0, 0)
Complexity
The time complexity is , and the space complexity is . The space complexity is .
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.