Formatted question description:

 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.

 The read function will only be called once for each test case.


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



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

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