158. 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.

/* 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
     */
    Queue<Character> queue = new LinkedList<>();
    public int read(char[] buf, int n) {

        int index = 0;
        while(!queue.isEmpty() && index < n){
            buf[index++] = queue.poll();
        }

        if(index == n) return n;

        char[] r4 = new char[4];

        while(index < n){
            int c = read4(r4);
            int i =0;
            for(; i< c && index < n; i++){
                buf[index++] = r4[i];
            }
            while(i < c){
                queue.offer(r4[i++]);
            }
            if(c < 4) break;// nothing to read from the stream/file.
        }

        return index;
    }
}