编程珠玑 Chapter2 字符串翻转算法

没办法，久没练过算法，只好用Java写了。

题目很简单：

Q: abcdefg, r=3

A: defgabc

代码一：借助StringBuffer的append来做。即一部分一部分来求逆，然后再“拼接”起来求逆。

public class Rotate {

/**
* Rotate a String by reversing it partly and then append them together to
* reverse.
*
* @param str
* @param num
* @return
*/
public static String rotateStr(String str, int num) {

StringBuffer tmpStr = new StringBuffer();

// Note: String#subString() doesn't contain the end index!
tmpStr = tmpStr.append(reverseStr(str.substring(0, num)));
tmpStr = tmpStr.append(reverseStr(str.substring(num, str.length())));

String result = reverseStr(tmpStr.toString());

return result;
}

/**
* Reverse a String.
*
* @param str
* @return
*/
private static String reverseStr(String str) {

int i = str.length() - 1;

StringBuffer result = new StringBuffer();

while (i >= 0) {
result.append(str.charAt(i));
i--;
}

return result.toString();
}

public static void main(String[] args) {

String test = "abcdefghijklmn";

System.out.println(rotateStr(test, 5));
}
}

代码二：不用append，对原字符串一部分一部分进行求逆操作，再整体求逆。求逆用的是围绕“中间点”交换字符来做的。

public class Rotateng {

/**
* Reverse part of a StringBuffer specified by start to end.
*
* @param str
* @param start
* @param end
* @return
*/
public static String reverse(String str, int start, int end) {

StringBuffer result = new StringBuffer(str);

// The middle of this part
int mid = (end + start) / 2;

for (int i = start; i <= mid; i++) {

// Calculate the location i should rotated to
int index = 2 * mid - i;

// If this part is even, index should be adjusted
if ((end - start + 1) % 2 == 0) {
index = index + 1;
}

char tmp = result.charAt(i);
result.setCharAt(i, result.charAt(index));
result.setCharAt(index, tmp);
}

return result.toString();
}

/**
* Rotate a String by reversing it partly but no append.
*
* @param str
* @param num
* @return
*/
public static String rotateStr(String str, int num) {

int end = str.length() - 1;

String result=reverse(str, 0, num-1);
result = reverse(result, num, end);
result = reverse(result, 0, end);

return result;
}

public static void main(String[] args) {

String test = "abcdefghijklmn";

System.out.println(rotateStr(test, 5));
}
}

两者效率差不太多，后者略高一点。（遍历一半的距离）