Read N Characters Given Read4

题目地址：
https://leetcode.com/problems/read-n-characters-given-read4/description/

题目：

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.

解题思路：
这道题就是用他给的api来做，每次尝试读取4个char放到temp里，然后做循环将temp里的char转移到globle的buf里。

代码：

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) {
        int p = 0;
        while(p < n){
            char[] temp = new char[4];
            int k = read4(temp);
            for(int i = 0; i < k && p < n; i++){
                buf[p] = temp[i];
                p++;
            }
            if(k < 4){
                return p;
            }
        }
        return p;
    }
}

Follow up:

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.

解题思路：

Think that you have 4 chars "a, b, c, d" in the file, and you want to call your function twice like this:

1. read(buf, 1); // should return 'a'
2. read(buf, 3); // should return 'b, c, d'

All the 4 chars will be consumed in the first call. So the tricky part of this question is how can you preserve the remaining 'b, c, d' to the second call.

p-> the pointer to the destination buffer

bufPointer -> the pointer to the temporary buf

k -> the real length for each read4 API

代码：

public class Solution extends Reader4 {
    /**
     * @param buf Destination buffer
     * @param n   Maximum number of characters to read
     * @return    The number of characters read
     */
    private int buffPtr = 0;
    private int k = 0;
    char[] temp = new char[4];
    public int read(char[] buf, int n) {
        int p = 0;
     while(p < n){
      if(buffPtr == 0){
          k = read4(temp);
            }
            if(k == 0){
             break;
            }
            while(p < n && buffPtr < k){
             buf[p++] = temp[buffPtr++];
            }
            if(buffPtr >= k){
             buffPtr = 0;
            }
        }
        return p;
    }

}