Question
Formatted question description: https://leetcode.ca/all/158.html
157 Read N Characters Given Read4
The API: int read4(char *buf) reads 4 characters at a time from a file.
The return value is the actual number of characters read.
For example, it returns 3 if there is only 3 characters left in the file.
By using the read4 API, implement the function
int read(char *buf, int n)
that reads n characters from the file.
Note:
The read function will only be called once for each test case.
Algorithm
Two variables readPos and writePos are used to record the position of reading and writing, i starts looping from 0 to n,
- If the read and write positions are the same at this time, call the read4 function, assign the result to writePos, and set readPos to zero,
- If writePos is zero, indicating that there is nothing in buf, return the current coordinate i. Then use the readPos position of the built-in buff variable to overwrite the i position of the input string buf,
- If the traversal is completed, return n
Code
Java
public class Read_N_Characters_Given_Read4 {
public class Solution extends Reader4 {
/**
* @param buf Destination buffer
* @param n Maximum number of characters to read
* @return The number of characters read
*/
public int read(char[] buf, int n) {
boolean isEof = false;
int charsRead = 0;
char[] buf4 = new char[4];
while (!isEof && charsRead < n) {
int size = read4(buf4);
if (size < 4) {
isEof = true;
}
if (charsRead + size > n) {
size = n - charsRead;
}
/*
arraycopy(Object src, int srcPos,
Object dest, int destPos,
int length);
*/
// if last iteration and size==0, then copy length==0, not over-copy
System.arraycopy(buf4, 0, buf, charsRead, size);
charsRead += size;
}
return charsRead;
}
}
private class Reader4 {
public int read4(char[] buf4) {
return 1; // stub
}
}
}