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Problem solution · Python

Read N Characters Given Read4

Read N Characters Given Read4: a Python solution using direct simulation. Learn the idea, check the complexity, and read the full code, with credit to walkccc LeetCode Solutions.

Technique
Direct simulation
Source
walkccc LeetCode Solutions
Length
32 lines
Start with the idea.

Try the problem first. If you get stuck, read the approach below, then write your own solution. The full code is at the bottom.

Approach

Direct simulation

For Read N Characters Given Read4, the implementation follows the problem’s operations directly while maintaining only the state needed for the next decision.

  1. Translate each rule into one explicit state update.
  2. Maintain the invariant after every processed item.
  3. Return the accumulated state once all relevant input has been handled.

Code notes

  • 32 lines of Python from the credited upstream file 157.py.
  • The implementation visibly relies on sequence storage.
  • No explicit loop blocks detected.

Complexity

Count the number and nesting of passes over the input, then include the maintained containers in the memory estimate.

Check the problem constraints before deciding whether this complexity will pass.

Source

Code and credit

This code comes from walkccc LeetCode Solutions by P.-Y. Chen (walkccc) and is used under the MIT licence.

Open problem ↗Original source ↗
Full codeRead N Characters Given Read4 · PythonPython
Use this to learn the idea, then write your own version.
"""The read4 API is already defined for you.  def read4(buf4: list[str]) -> int: # Below is an example of how the read4 API can be called.file = File("abcdefghijk") # File is "abcdefghijk", initially file pointer (fp) points to 'a'buf4 = [' '] * 4 # Create buffer with enough space to store charactersread4(buf4) # Read4 returns 4. Now buf = ['a','b','c','d'], fp points to 'e'read4(buf4) # Read4 returns 4. Now buf = ['e','f','g','h'], fp points to 'i'read4(buf4) # Read4 returns 3. Now buf = ['i','j','k',...], fp points to end of file"""  class Solution:  def read(self, buf: list[str], n: int) -> int:    buf4 = [' '] * 4    i4 = 0  # buf4's index    n4 = 0  # buf4's size    i = 0  # buf's index     while i < n:      if i4 == n4:  # All the characters in the buf4 are consumed.        i4 = 0  # Reset the buf4's index.        n4 = read4(buf4)  # Read <= 4 characters from the file to the buf4.        if n4 == 0:  # Reach the EOF.          return i      buf[i] = buf4[i4]      i += 1      i4 += 1     return i

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