LC157. 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.
一开始没读懂题意，这题是说给一个read4的API，它能够从文件中读取4个char并存到buf这个地址中，并返回读取到的char数量。如果文件中不到4个char，那么就读取剩下的char。现在利用这个API实现一个readn的API，对一个文件只调用一次。
/* The read4 API is defined in the parent class Reader4.
int read4(char[] buf); */
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) {
char[] buffer = new char[4];
int pos = 0;
while (pos < n) {
int temp = read4(buffer);
int next = Math.min(n - pos, temp);
for (int i= 0; i < next; i++) {
buf[pos + i] = buffer[i];
}
pos += next;
if(temp < 4) break;
}
return pos;
}
}
题意理解了之后利用pos指针记录读取的位置，最后返回读取的总数即可。
LC158. Read N Characters Given Read4 II - Call multiple times
这题和刚才的区别是对于一个文件可以调用多次readn，也就是说假设文件长度足够，第一次n = 1，第二次n = 2，总读取的char数目为3，分别返回1，2.
/* The read4 API is defined in the parent class Reader4.
int read4(char[] buf); */
public class Solution extends Reader4 {
/**
* @param buf Destination buffer
* @param n Maximum number of characters to read
* @return The number of characters read
*/
// it can be called for one buffer for several times
// so used private static variables
private int buffPtr = 0;
private int buffCnt = 0;
private char[] buffer = new char[4];
public int read(char[] buf, int n) {
int count = 0; // count of number read from buffer
while(count < n){
if(buffPtr == 0) buffCnt = read4(buffer);
while(count < n && buffPtr < buffCnt) buf[count++] = buffer[buffPtr++];
if(buffCnt == buffPtr) buffPtr = 0; // if this read4 has all been copied
if(buffCnt < 4) break;
}
return count;
}
}
因为每次读取起始位置的变化需要利用全局变量来记录。不断利用read4读取，直到read4读取 < 4或读取总数< n停止。