158 Read N Characters Given Read4 II - Call multiple times

158. Read N Characters Given Read4 II - Call multiple times​
1. Question
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 theread4API, implement the functionint read(char *buf, int n)that readsncharacters from the file.
Note:
Thereadfunction may be called multiple times.
2. Implementation
思路:
(1) 这题和157不同的是read4()可能被call很多次，这意味着之前call read4()会有一些数据保留在buffer里未读到。举个例子，比如输入数据是 s = "abcde"， 第一次我们call read(s, 1), 我们只要读a，但里面read4()实际上读"abcd", bcd还在buffer未被读取。如果我们下一次call read4(s, 4), 我们要读取"bcde",要先把buffer之前保留的数据读取。
(2) 解决方法是用一个全局数组buffer[] 保留每次read4()返回的数据，同时bufferIndex记录buffer上次读到的位置， 只有当bufferIndex等于readBytes时，之前的buffer的数据才读完。
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/* The read4 API is defined in the parent class Reader4.
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      int read4(char[] buf); */
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​
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public class Solution extends Reader4 {
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    /**
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     * @param buf Destination buffer
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     * @param n   Maximum number of characters to read
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     * @return    The number of characters read
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     */
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​
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    int bufferIndex = 0;
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    int readBytes = 0;
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    char[] buffer = new char[4];
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​
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    public int read(char[] buf, int n) {
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        int index = 0;
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​
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        while (index < n) {
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            if (bufferIndex < readBytes) {
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                buf[index++] = buffer[bufferIndex++];
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            }
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            else {
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                readBytes = read4(buffer);
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                bufferIndex = 0;
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                if (readBytes == 0) {
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                    break;
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                }
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            }
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        }
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        return index;
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    }
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}
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3. Time & Space Complexity
时间复杂度O(n), 空间复杂度O(1)

157 Read N Characters Given Read4

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Contents

158. Read N Characters Given Read4 II - Call multiple times

1. Question

2. Implementation

3. Time & Space Complexity

158. Read N Characters Given Read4 II - Call multiple times​

1. Question

2. Implementation

3. Time & Space Complexity

158. Read N Characters Given Read4 II - Call multiple times