Leetcode: Read N Characters Given Read4 II - Call multiple times
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 may be called multiple times. Show Tags Have you met this question in a real interview?
这道题跟I不一样在于,read函数可能多次调用,比如read(buf,23)之后又read(buf, 25), 第一次调用时的buffer还没用完,还剩一个char在buffer里,第二次拿出来接着用,这样才能保证接着上次读的地方继续往下读。
1. 所以应该设置这4个char的buffer为instance variable(实例变量),这样每次调用read函数后buffer还剩的元素可以被保存下来,供给下一次读取
2. 那么下一次调用read函数时候,怎么知道上一次buffer里面还剩不剩未读元素呢?我们有oneRead(一次读到buffer里的char数),actRead(实际被读取的char数),oneRead-actRead就是还剩在buffer里的char数。通常oneRead == actRead, 只有当n-haveRead < oneRead时,才不等,这就是上一次调用read结束的时候。所以只要调用read函数发现oneRead != 0 时,就说明上一次调用read还剩了元素在buffer里,先读完这个,再调用read4读新的。oneRead也需要是instance varaible
3. 还需要设置一个offset: Use to keep track of the offset index where the data begins in the nextread call. The buffer could be read partially (due to constraints of reading upto n bytes) and therefore leaving some data behind.
|<--buffer-->|
// |_________________________|
// |<---offset---> |<----oneRead--->
上图所示为一次read最后的情况,offset部分其实就是actRead的部分,oneRead = oneRead - actRead, 就剩下了右边一部分在buffer里没有读,下一次read函数调用,发现oneRead>0, 说明上一次read还剩了一部分没有读。oneRead表示的其实就是上一次读剩下的char数,offset表示这一次读应该开始的位置
其实上图的oneRead不一定会充满整个右边部分的,有可能上一次读oneRead根本没有读满整个buffer。 所以oneRead+offset并不一定等于整个buffer。这也就是为什么我们一定要用两个变量oneRead\offset的原因,因为oneRead并不一定=4-offset
1 /* The read4 API is defined in the parent class Reader4. 2 int read4(char[] buf); */ 3 4 public class Solution extends Reader4 { 5 /** 6 * @param buf Destination buffer 7 * @param n Maximum number of characters to read 8 * @return The number of characters read 9 */ 10 private char buffer = new char[4]; 11 private int oneRead = 0; 12 private int offset = 0; 13 14 public int read(char[] buf, int n) { 15 boolean lessthan4 = false; 16 int haveRead = 0; 17 while (!lessthan4 && haveRead < n) { 18 if (oneRead == 0) { 19 oneRead = read4(buffer); 20 lessthan4 = oneRead < 4; 21 } 22 int actRead = Math.min(n-haveRead, oneRead); 23 for (int i=0; i<actRead; i++) { 24 buf[haveRead+i] = buffer[offset+i]; 25 } 26 oneRead -= actRead; 27 offset = (offset + actRead) % 4; 28 haveRead += actRead; 29 } 30 return haveRead; 31 } 32 }