【剑指offer】替换空格

今天药忘吃喽~ 2022-06-16 09:11 169阅读 0赞

## 题目描述 ##

请实现一个函数，将一个字符串中的空格替换成“%20”。例如，当字符串为We Are Happy.则经过替换之后的字符串为We%20Are%20Happy。

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public class TestReplaceSpace { 
    
        public static void main(String[] args) {
            StringBuffer str = new StringBuffer("We Are Happy");
            System.out.print(replaceSpace2(str));
        }
    
        public static String replaceSpace(StringBuffer str) {
            if (str == null) {
                return null;
            }
            StringBuilder sb = new StringBuilder();// 新字符串
            for (int i = 0; i < str.length(); i++) {
                if (str.charAt(i) == ' ') {
                    sb.append("%20");
                } else {
                    sb.append(str.charAt(i));
                }
            }
            return sb.toString();
        }
    
        public static String replaceSpace2(StringBuffer str) {
            if (str == null) {
                return null;
            }
            int spaceNum = 0; // 原字符串空格个数
            for (int i = 0; i < str.length(); i++) {
                if (str.charAt(i) == ' ') {
                    spaceNum++;
                }
            }
            int newLength = spaceNum * 2 + str.length(); // 新长度 = 原字符串空格个数*2 + 原长
            int oldEnd = str.length() - 1; // 原字符串尾指针
            int newEnd = newLength - 1; // 新字符串尾指针
            str.setLength(newLength); // 扩充原字符串长度为新长度
            while (oldEnd >= 0) {
                if (str.charAt(oldEnd) == ' ') {
                    str.setCharAt(newEnd--, '0');
                    str.setCharAt(newEnd--, '2');
                    str.setCharAt(newEnd--, '%');
                    oldEnd--;
                    continue;
                } else {
                    str.setCharAt(newEnd--, str.charAt(oldEnd--));
                }
            }
            return str.toString();
        }
    }

replaceSpace() 方法新建了一个 StringBuilder 对象来复制原字符串的内容并将里面的空格替换成”%20”；

replaceSpace2() 方法则先计算原字符串里的空格，扩充原字符串长度，从后往前复制原字符串里的值，并替换空格。

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StringBuffer 的 setLength 函数用法如下：

> **void java.lang.StringBuffer.setLength(int newLength)**
> 
> Sets the length of the character sequence. The sequence is changed to a new character sequence whose length is specified by the argument. For every nonnegative index k less than newLength, the character at index k in the new character sequence is the same as the character at index k in the old sequence if k is less than the length of the old character sequence; otherwise, it is the null character ‘\\u005Cu0000’. In other words, if the newLength argument is less than the current length, the length is changed to the specified length.  
> If the newLength argument is greater than or equal to the current length, sufficient null characters (‘\\u005Cu0000’) are appended so that length becomes the newLength argument.  
> The newLength argument must be greater than or equal to 0.

该函数没有返回值。  
若参数 newLength 大于或等于原长度，原来字符串里的字符位置不变，新多出来的位置用 null 字符(‘\\u005Cu0000’)填充；  
若参数 newLength 小于原长度，就相当于在原字符串上截取 newLength 长度的字符串。  
newLength >= 0