Question

Formatted question description: https://leetcode.ca/all/157.html

 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.

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

Algorithm

Read every 4, and then judge the results,

If it is 0, it means that the buf has been read at this time, and the loop is jumped out, and the smaller value of res and n is directly returned.
Otherwise, keep reading until n characters are read, the loop ends, and finally the smaller value of res and n is returned

Code

Java

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
        }
    }
}

// OJ: https://leetcode.com/problems/read-n-characters-given-read4/
// Time: O(N)
// Space: O(1)
// Forward declaration of the read4 API.
int read4(char *buf);
class Solution {
public:
    /**
     * @param buf Destination buffer
     * @param n   Number of characters to read
     * @return    The number of actual characters read
     */
    int read(char *buf, int n) {
        int cnt = 0;
        while (cnt < n) {
            int x = read4(buf + cnt);
            if (!x) break;
            cnt += x;
        }
        cnt = min(cnt, n);
        buf[cnt] = '\0';
        return cnt;
    }
};

# The read4 API is already defined for you.
# @param buf, a list of characters
# @return an integer
# def read4(buf):

class Solution(object):
  def read(self, buf, n):
    """
    :type buf: Destination buffer (List[str])
    :type n: Maximum number of characters to read (int)
    :rtype: The number of characters read (int)
    """
    cnt = 0
    tmp = [""] * 4
    while cnt < n:
      r = read4(tmp)
      if r == 0:
        break
      for i in range(min(r, n - cnt)):
        buf[cnt] = tmp[i]
        cnt += 1
    return cnt

157 - Read N Characters Given Read4

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