157. Read N Characters Given Read4

Problem Statement

link: LeetCode.cn LeetCode 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.

Example:

Input: ["LRUCache", "put", "put", "get", "put", "get"，"put", "get", "get", "get"][[2], [1, 1], [2, 2], [1], [3, 3], [2], [4, 4], [1], [3], [4]] Output: [null, null, null, 1, null, -1, null, -1, 3, 4]

Constraints: 1 <= file.length <= 500 file consist of English letters and digits. 1 <= n <= 1000

Solution Approach

For the solution, you can summarize it as: “Iteratively call read4 to read characters in chunks of four, then copy these characters into the main buffer until the desired number of characters is read or the file ends.

Algorithm

1. Chunk Reading: Use the read4 method to read up to four characters at a time.
2. Buffer Copying: Transfer the characters read by read4 into the main buffer.
3. Loop Until Complete: Continue this process until the desired number of characters is read or the end of the file is reached.

Implement

    class Solution:
        def read(self, buf, n):
            """
            :type buf: Destination buffer (List[str])
            :type n: Number of characters to read (int)
            :rtype: The number of actual characters read (int)
            """
            bi = 0
            for _ in range(0, n, 4):
                tmp = [None] * 4 #this is buf4
                cur_len = read4(tmp)
                for j in range(cur_len):
                    buf[bi] = tmp[j]
                    bi += 1
            return min(bi, n)

    
    

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