面试题2：替换空格

本题来自《剑指offer 名企面试官精讲典型编程题》面试题4。

题目4：请实现一个函数，把字符串中的每个空格替换成"%20"。例如：输入"hello world."，则输出"hello%20world."。

在网络编程中，如果URL参数中含有特殊字符，如空格，'#'等，可能导致服务端无法获得正确的参数值。我们需要将特殊符号转换为服务器可以识别的字符。转换规则为在'%'后面跟上ASCII码的两位十六进制。空格可以被替换为"%20"，'#'可以被替换为"%23"。

时间复杂度为O(n)的解法：

可以先遍历一次字符串，统计出字符串中的空格总数，并由此计算出替换之后的字符串总长度。每替换一个空格，长度增加2。替换后的字符串长度等于原来的长度加上空格数乘以2。

准备两个指针P1，P2从后往前开始复制替换。P1指向原字符串末尾，P2指向替换之后的字符串末尾。接下来我们向前移动指针P1，逐步将它指向的字符复制到P2指向的位置，直到碰到第一个空格为止。碰到第一个空格后，P1向前移动一格，P2之前插入"%20"，同时把P2向前移动3格。当P1和P2指向同一个位置，表明所有空格替换完毕。

以下为操作示意图：

package array;

/**
* @author WuPing
* @version 2016年4月8日 上午11:05:59
*/

public class ReplaceBlank {

public static void replaceBlank(char strings[], int length) {
// length为字符数组string的总容量
if (strings != null && length > 0) {
int originalLength = 0; // 字符数组实际长度
int numberOfBlank = 0; // 字符数组中的空格数
int i = 0;
while (strings[i] != '\0') {
++originalLength;

if (strings[i] == ' ')
++numberOfBlank;

++i;
}

// newLength为把空格替换成'%20'之后的长度
int newLength = originalLength + numberOfBlank * 2;
if (newLength > length)
return;

int indexOfOriginal = originalLength; // 字符数组起始扫描索引
int indexOfNew = newLength; // 替换空格后的字符数组起始扫描索引
while (indexOfOriginal >= 0 && indexOfNew > indexOfOriginal) {
// 从后向前扫描，遇到空格则替换成'%20'，否则直接复制字符
if (strings[indexOfOriginal] == ' ') {
strings[indexOfNew--] = '0';
strings[indexOfNew--] = '2';
strings[indexOfNew--] = '%';
} else {
strings[indexOfNew--] = strings[indexOfOriginal];
}

--indexOfOriginal;
}
}
}

//空格在字符串中间
public static void Test1(int length) {
char[] test1Strings = new char[length];
test1Strings[0] = 'h';
test1Strings[1] = 'e';
test1Strings[2] = 'l';
test1Strings[3] = 'l';
test1Strings[4] = 'o';
test1Strings[5] = ' ';
test1Strings[6] = 'w';
test1Strings[7] = 'o';
test1Strings[8] = 'r';
test1Strings[9] = 'l';
test1Strings[10] = 'd';
test1Strings[11] = '\0'; // 模拟字符串结束符

System.out.println("Test1 begin: ");
System.out.print("替换空格前的字符数组：");
for (int i = 0; i < test1Strings.length; i++) {
if (test1Strings[i] == '\0') {
break;
}
System.out.print(test1Strings[i]);
}
System.out.println();

replaceBlank(test1Strings, length);

System.out.print("替换空格后的字符数组：");
for (int i = 0; i < test1Strings.length; i++) {
if (test1Strings[i] == '\0') {
break;
}
System.out.print(test1Strings[i]);
}
System.out.println();
}

//空格在字符串开头
public static void Test2(int length) {
char[] test2Strings = new char[length];
test2Strings[0] = ' ';
test2Strings[1] = 'h';
test2Strings[2] = 'e';
test2Strings[3] = 'l';
test2Strings[4] = 'l';
test2Strings[5] = 'o';
test2Strings[6] = 'w';
test2Strings[7] = 'o';
test2Strings[8] = 'r';
test2Strings[9] = 'l';
test2Strings[10] = 'd';
test2Strings[11] = '\0';  // 模拟字符串结束符

System.out.println("Test2 begin: ");
System.out.print("替换空格前的字符数组：");
for (int i = 0; i < test2Strings.length; i++) {
if (test2Strings[i] == '\0') {
break;
}
System.out.print(test2Strings[i]);
}
System.out.println();

replaceBlank(test2Strings, length);

System.out.print("替换空格后的字符数组：");
for (int i = 0; i < test2Strings.length; i++) {
if (test2Strings[i] == '\0') {
break;
}
System.out.print(test2Strings[i]);
}
System.out.println();
}

//空格在字符串末尾
public static void Test3(int length) {
char[] test3Strings = new char[length];
test3Strings[0] = 'h';
test3Strings[1] = 'e';
test3Strings[2] = 'l';
test3Strings[3] = 'l';
test3Strings[4] = 'o';
test3Strings[5] = 'w';
test3Strings[6] = 'o';
test3Strings[7] = 'r';
test3Strings[8] = 'l';
test3Strings[9] = 'd';
test3Strings[10] = ' ';
test3Strings[11] = '\0';  // 模拟字符串结束符

System.out.println("Test3 begin: ");
System.out.print("替换空格前的字符数组：");
for (int i = 0; i < test3Strings.length; i++) {
if (test3Strings[i] == '\0') {
break;
}
System.out.print(test3Strings[i]);
}
System.out.println();

replaceBlank(test3Strings, length);

System.out.print("替换空格后的字符数组：");
for (int i = 0; i < test3Strings.length; i++) {
if (test3Strings[i] == '\0') {
break;
}
System.out.print(test3Strings[i]);
}
System.out.println();
}

//字符串中有连续两个空格
public static void Test4(int length) {
char[] test4Strings = new char[length];
test4Strings[0] = 'h';
test4Strings[1] = 'e';
test4Strings[2] = 'l';
test4Strings[3] = 'l';
test4Strings[4] = 'o';
test4Strings[5] = ' ';
test4Strings[6] = ' ';
test4Strings[7] = 'w';
test4Strings[8] = 'o';
test4Strings[9] = 'r';
test4Strings[10] = 'l';
test4Strings[11] = 'd';
test4Strings[12] = '\0';  // 模拟字符串结束符

System.out.println("Test4 begin: ");
System.out.print("替换空格前的字符数组：");
for (int i = 0; i < test4Strings.length; i++) {
if (test4Strings[i] == '\0') {
break;
}
System.out.print(test4Strings[i]);
}
System.out.println();

replaceBlank(test4Strings, length);

System.out.print("替换空格后的字符数组：");
for (int i = 0; i < test4Strings.length; i++) {
if (test4Strings[i] == '\0') {
break;
}
System.out.print(test4Strings[i]);
}
System.out.println();
}

//字符数组为null
public static void Test5() {
System.out.print("Test5 begin: ");
replaceBlank(null, 0);
System.out.println("passed!");
}

//字符串为空串
public static void Test6(int length) {
char[] test6Strings = new char[length];
test6Strings[0] = '\0';  // 模拟字符串结束符

System.out.println("Test6 begin: ");
System.out.print("替换空格前的字符数组：");
for (int i = 0; i < test6Strings.length; i++) {
if (test6Strings[i] == '\0') {
break;
}
System.out.print(test6Strings[i]);
}
System.out.println();

replaceBlank(test6Strings, length);

System.out.print("替换空格后的字符数组：");
for (int i = 0; i < test6Strings.length; i++) {
if (test6Strings[i] == '\0') {
break;
}
System.out.print(test6Strings[i]);
}
System.out.println();
}

//字符串为一个空格
public static void Test7(int length) {
char[] test7Strings = new char[length];
test7Strings[0] = ' ';
test7Strings[1] = '\0';  // 模拟字符串结束符

System.out.println("Test7 begin: ");
System.out.print("替换空格前的字符数组：");
for (int i = 0; i < test7Strings.length; i++) {
if (test7Strings[i] == '\0') {
break;
}
System.out.print(test7Strings[i]);
}
System.out.println();

replaceBlank(test7Strings, length);

System.out.print("替换空格后的字符数组：");
for (int i = 0; i < test7Strings.length; i++) {
if (test7Strings[i] == '\0') {
break;
}
System.out.print(test7Strings[i]);
}
System.out.println();
}

//字符串中没有空格
public static void Test8(int length) {
char[] test8Strings = new char[length];
test8Strings[0] = 'h';
test8Strings[1] = 'e';
test8Strings[2] = 'l';
test8Strings[3] = 'l';
test8Strings[4] = 'o';
test8Strings[5] = 'w';
test8Strings[6] = 'o';
test8Strings[7] = 'r';
test8Strings[8] = 'l';
test8Strings[9] = 'd';
test8Strings[10] = '\0'; // 模拟字符串结束符

System.out.println("Test8 begin: ");
System.out.print("替换空格前的字符数组：");
for (int i = 0; i < test8Strings.length; i++) {
if (test8Strings[i] == '\0') {
break;
}
System.out.print(test8Strings[i]);
}
System.out.println();

replaceBlank(test8Strings, length);

System.out.print("替换空格后的字符数组：");
for (int i = 0; i < test8Strings.length; i++) {
if (test8Strings[i] == '\0') {
break;
}
System.out.print(test8Strings[i]);
}
System.out.println();
}

//字符串中全是空格
public static void Test9(int length) {
char[] test9Strings = new char[length];
test9Strings[0] = ' ';
test9Strings[1] = ' ';
test9Strings[2] = ' ';
test9Strings[3] = ' ';
test9Strings[4] = '\0'; // 模拟字符串结束符

System.out.println("Test9 begin: ");
System.out.print("替换空格前的字符数组：");
for (int i = 0; i < test9Strings.length; i++) {
if (test9Strings[i] == '\0') {
break;
}
System.out.print(test9Strings[i]);
}
System.out.println();

replaceBlank(test9Strings, length);

System.out.print("替换空格后的字符数组：");
for (int i = 0; i < test9Strings.length; i++) {
if (test9Strings[i] == '\0') {
break;
}
System.out.print(test9Strings[i]);
}
System.out.println();
}

public static void main(String[] args) {
int length = 100;
Test1(length);
Test2(length);
Test3(length);
Test4(length);
Test5();
Test6(length);
Test7(length);
Test8(length);
Test9(length);
}

}

View Code
程序运行结果如下图：