LeetCode 383: Ransom Note — Python Solution

Solve LeetCode 383: Ransom Note in Python with a frequency consumption approach. The key is to make the state invariant explicit, so the implementation and complexity follow naturally.

This guide paraphrases the task and does not reproduce LeetCode’s prompt. Use the official page for the complete statement, examples, constraints, and submission runner.

What the problem is testing

Count available magazine characters and decrement for every requested character, failing when a count is exhausted.

Algorithm

1. Count available magazine characters and decrement for every requested character, failing when a count is exhausted.
2. Maintain this invariant: Counts represent unused magazine characters after satisfying the processed prefix.
3. Continue until every input item or reachable state has been resolved, then return the accumulated result.

Python solution

from collections import Counter, defaultdict, deque, OrderedDict
import random

class Solution:
    def canConstruct(self, ransomNote, magazine):
        available = Counter(magazine)
        for char in ransomNote:
            available[char] -= 1
            if available[char] < 0: return False
        return True

Why this is correct

The proof follows the maintained state: Counts represent unused magazine characters after satisfying the processed prefix. Each iteration preserves that claim while permanently resolving at least one position, node, interval, or search state. When the loop or recursion ends, every candidate required by the problem has therefore been included or ruled out, so the returned value is correct.

Complexity

O(n+m) time and O(k) space. The stated auxiliary space excludes the returned output unless the output is the data structure being built.

Edge cases

Repeated requested letters require separate available copies.

Tested reference code

This implementation is included in the site’s downloadable 100-solution Python library. The complete suite compiles every solution and runs a behavioral assertion for every problem before publication.

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