Skip to content

158. Read N Characters Given read4 II - Call Multiple Times

Given a file and assume that you can only read the file using a given method read4, implement a method read to read n characters. Your method read may be called multiple times.

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:

img

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 may be called multiple times.
  • Please remember to RESET your class variables declared in Solution, as static/class variables are persisted across multiple test cases. Please see here for more details.
  • You may assume the destination buffer array, buf, is guaranteed to have enough space for storing n characters.
  • It is guaranteed that in a given test case the same buffer buf is called by read.

Example 1:

Input: file = "abc", queries = [1,2,1]
Output: [1,2,0]
Explanation: The test case represents the following scenario:
File file("abc");
Solution sol;
sol.read(buf, 1); // After calling your read method, buf should contain "a". We read a total of 1 character from the file, so return 1.
sol.read(buf, 2); // Now buf should contain "bc". We read a total of 2 characters from the file, so return 2.
sol.read(buf, 1); // We have reached the end of file, no more characters can be read. So return 0.
Assume buf is allocated and guaranteed to have enough space for storing all characters from the file.

Example 2:

Input: file = "abc", queries = [4,1]
Output: [3,0]
Explanation: The test case represents the following scenario:
File file("abc");
Solution sol;
sol.read(buf, 4); // After calling your read method, buf should contain "abc". We read a total of 3 characters from the file, so return 3.
sol.read(buf, 1); // We have reached the end of file, no more characters can be read. So return 0.

Constraints:

  • 1 <= file.length <= 500
  • file consist of English letters and digits.
  • 1 <= queries.length <= 10
  • 1 <= queries[i] <= 500

Solution:

/**
 * The read4 API is defined in the parent class Reader4.
 *     int read4(char[] buf4); 
 */

/*
    [a, b, c, d, e]

    buff
     0 1 2 3 
    [ , , , ]

*/

public class Solution extends Reader4 {
    /**
     * @param buf Destination buffer
     * @param n   Number of characters to read
     * @return    The number of actual characters read
     */

    private int buffPtr = 0; 
    // This is a pointer to keep track of the current position in the `buff` array.
    // It helps to know which character in `buffer` should be read next;
    private int buffCnt = 0;
    // This keeps track of the number of characters actually returned by the `read4`
    // method. It might be less than 4 if the end of the file is reached.
    private char[] buff = new char[4];
    // This is a temporary buffer of size 4 that stores characters read from the `read4`
    // method
    public int read(char[] buf, int n) { // " a b c"
        int ptr = 0; // --> number of char we read
        while (ptr < n){
            if (buffPtr == 0){
                buffCnt = read4(buff); // it will update the buff and return the point
            }

            if (buffCnt == 0){
                break; // nothing to read
            }

            while(ptr < n && buffPtr < buffCnt){
                buf[ptr] = buff[buffPtr];
                ptr++;
                buffPtr++; 
            }

            if (buffPtr >= buffCnt){
                buffPtr = 0;
            }

        }

        return ptr;
    }
}

做过类似

嗯...

又是看不懂题系列

158 是 157 的follow up