158. Read N Characters Given Read4 II

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.

Example 1: 

Given buf = "abc"
read("abc", 1) // returns "a"
read("abc", 2); // returns "bc"
read("abc", 1); // returns ""

Example 2: 

Given buf = "abc"
read("abc", 4) // returns "abc"
read("abc", 1); // returns ""

reference: 

https://leetcode.com/problems/read-n-characters-given-read4-ii-call-multiple-times/discuss/49598/A-simple-Java-code

https://segmentfault.com/a/1190000003794420

因为可以调用多次，每一次调用的时候都需要检查上一次是否有未读完的字符串，从上一次未读完的接着读。

e.g.
Given buf = "abc" tmp = {0,0,0,0}, count = 0, ptr = 0
read("abc", 1); // returns "a"
-> index = 0, n = 1, count = read4(tmp) = 4, buf[0]=tmp[0] -> index = 1, ptr = 1, return index = 1
read("abc", 2); // returns "bc"
-> index = 0, n = 2, count = read4(tmp) = 4, buf[0]=tmp[1],buf[1]=tmp[2] -> index = 2, ptr = 3, return index = 2
read("abc", 1); // returns ""
-> index = 0, n = 1, count = read4(tmp) = 4, buf[0]=tmp[3] -> index = 1, ptr = 4 -> ptr = 0, return index = 1

time: O(N), space: O(1)

/* 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
     */
    private char[] tmp = new char[4];
    private int count = 0;
    private int ptr = 0;
    
    public int read(char[] buf, int n) {
        int index = 0;
        while(index < n) {
            if(ptr == 0) {
                count = read4(tmp);
            }
            if(count == 0) break;
            while(index < n && ptr < count) {
                buf[index++] = tmp[ptr++];
            }
            if(ptr == count) ptr = 0;
        }
        return index;
    }
}