Read n characters given read4

Array Simulation Interactive

42.5% acceptance

Mar 31, 2026

604

3605

Given a file and assume that you can only read the file using a given method read4, implement a method to read n characters. Method read4: The API read4 reads four consecutive characters from file, then writes those characters into the buffer array buf4. The return value is the number of actual characters read. Note that read4() has its own file pointer, much like FILE *fp in C. Definition of read4: Parameter: char[] buf4 Returns: int buf4[] is a destination, not a source. The results from read4 will be copied to buf4[]. Below is a high-level example of how read4 works: File file("abcde"); // File is "abcde", initially file pointer (fp) points to 'a' char[] buf4 = new char[4]; // Create buffer with enough space to store characters read4(buf4); // read4 returns 4. Now buf4 = "abcd", fp points to 'e' read4(buf4); // read4 returns 1. Now buf4 = "e", fp points to end of file read4(buf4); // read4 returns 0. Now buf4 = "", fp points to end of file Method read: By using the read4 method, implement the method read that reads n characters from file and store it in the buffer array buf. Consider that you cannot manipulate file directly. The return value is the number of actual characters read. Definition of read: Parameters: char[] buf, int n Returns: int buf[] is a destination, not a source. You will need to write the results to buf[]. Note: Consider that you cannot manipulate the file directly. The file is only accessible for read4 but not for read. The read function will only be called once for each test case. You may assume the destination buffer array, buf, is guaranteed to have enough space for storing n characters.

Solution

Rust

Time O(n²)

Space O(1)

LeetCode

solution.rs

/**
 * The read4 API is defined as.
 *     fn read4(&self,buf4: &mut [char]) -> i32;
 * You can call it using self.read4(buf4)
 */

impl Solution {
  pub fn read(&self, buf: &mut [char], n: i32) -> i32 {
    let mut total = 0i32;
    let mut buf4 = [' '; 4];
    loop {
      let count = self.read4(&mut buf4);
      let to_copy = count.min(n - total);
      for i in 0..to_copy as usize {
        buf[total as usize + i] = buf4[i];
      }
      total += to_copy;
      if count < 4 || total == n {
        break;
      }
    }
    total
  }
}