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 theread4API, implement the functionint read(char *buf, int n)that readsncharacters from the file.

Note:
Thereadfunction will only be called once for each test case.

分析

图例

代码

/* 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) {
     for(int i = 0; i < n; i = i + 4){
            char[] tmp = new char[4];
            // 将数据读入临时数组
            int len = read4(tmp);
            // 将临时数组拷贝至buf数组，这里拷贝的长度是本次读到的个数和剩余所需个数中较小的
         //src − This is the source array.  srcPos − This is the starting position in the source array. dest − This is the destination array.destPos − This is the starting position in the destination data.length − This is the number of array elements to be copied.
            System.arraycopy(tmp, 0, buf, i, Math.min(len, n - i));

            // 如果读不满4个，说明已经读完了，返回总所需长度和目前已经读到的长度的较小的
            if(len < 4) return Math.min(i + len, n);
        }
        // 如果循环内没有返回，说明读取的字符是4的倍数
        return n;
}
}