[LeetCode]Read N Characters Given Read4

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.

分析

這道題沒什麼特殊的算法跟數據結構，注意越界就能夠了。至於越界主要包含兩種狀況，一是讀到文件結尾，沒得讀了。而是還沒讀到文件結尾，可是已經達到要求的n個字符了，此時應該中止繼續讀。處理好這兩種狀況便可。

這道題Follow up能夠是屢次讀，也是LeetCode原題，即Read N Characters Given Read4 II - Call multiple times, 具體見後文。

複雜度

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

代碼

/* 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
     */
    public int read(char[] buf, int n) {
        int i = 0;
        char[] buffer = new char[4];
        while (i < n) {
            int bufCount = read4(buffer);
            int j = 0;
            while (j < bufCount && i < n) {
                buf[i++] = buffer[j++];
            }
            if (bufCount != 4)
                break;
        }
        return i;
    }
}

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.

分析

屢次讀與一次讀的主要不一樣在於read4()函數中的buffer至關於global的，每次read()的時候前面read4()裏讀進buffer的剩下來字符的還要繼續用，不夠才又調用read4()往buffer裏新增長內容供read讀取。

因此咱們只要存一個global的針對read4()的buffer的起始位置和終止位置便可。每次read()先讀上次buffer裏沒讀完的字符，不夠才又調用read4(). 固然，越界問題仍是得注意，跟上一道題同樣。

複雜度

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

代碼

/* The read4 API is defined in the parent class Reader4.
      int read4(char[] buf); */

public class Solution extends Reader4 {
    char[] buffer = new char[4];
    int start = 0; // inclusive
    int end = 0; // exclusive
    /**
     *@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 i = 0;
        while (i < n) {
            
            // 以前read4()讀進buffer裏的字符已所有讀完
            if (start == end) {
                end = read4(buffer);
                start = 0;
            }
            
            // 依次把buffer裏的字符讀進buf裏
            while (i < n && start < end) {
                buf[i++] = buffer[start++];
            }
            
            // 判斷是否到達文件末尾，是的話跳出循環
            if (end != 4)
                break;
        }
        return i;
    }
}