157 Read N Characters Given Read4
1. Question
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 theread4
API, implement the functionint read(char *buf, int n)
that readsncharacters from the file.
Note:
Theread
function will only be called once for each test case.
2. Implementation
思路: 这题要我们利用一个已有的函数read4()实现一个自定义的函数read(), 和read4()不同的是,read()里有一个参数n,定义我们读多少byte的数据,而read4()只会一次读4个byte。这里主要的考点是edge cases. 如果read4返回的值是小于4,说明是end of file。同时为了保证我们读取不超过 n个byte的数据,每次将数据copy到buffer时,我们都要取n - index和size之间较小的那个数。
/* 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) {
boolean eof = false;
char[] buffer = new char[4];
int index = 0;
while (!eof && index < n) {
int size = read4(buffer);
if (size < 4) {
eof = true;
}
int bytes = Math.min(n - index, size);
for (int i = 0; i < bytes; i++) {
buf[index++] = buffer[i];
}
}
return index;
}
}
3. Time & Space Complexity
时间复杂度O(n), 空间复杂度O(1)
Last updated
Was this helpful?