Sword finger offer question brushing record - offer 05 Replace spaces

Please implement a function , Put the string s Replace each space in with "%20".

Example 1：

Input ：s = "We are happy."
Output ："We%20are%20happy."

It is worth noting that ,java The string in is an immutable character sequence , Therefore, we need to find a way to change the content .

Method 1 ： A character array （ Official answer ）
Because every replacement from 1 Characters become 3 Characters , Using character arrays makes it easy to replace . Set the length of the character array to s The length of the 3 times , This ensures that the character array can hold all the replaced characters .

get s The length of length
Create a character array array, The length of length * 3
initialization size by 0,size Represents the length of the replaced string
Traversing the string from left to right s
get s The current character of c
If the characters c Is a space , Then order array[size] = '%',array[size + 1] = '2',array[size + 2] = '0', And will size The value of the add 3
If the characters c It's not a space , Then order array[size] = c, And will size The value of the add 1
After the traversal ,size The value of is equal to the length of the replaced string , from array Before size Create a new string of characters , And return the new string

    public String replaceSpace(String s) {
        int length = s.length();
        char[] array = new char[length * 3];
        int size = 0;
        for (int i = 0; i < length; i++) {
            char c = s.charAt(i);
            if (c == ' ') {
                array[size++] = '%';
                array[size++] = '2';
                array[size++] = '0';
            } else {
                array[size++] = c;
            }
        }
        String newStr = new String(array, 0, size);
        return newStr;
    }

Method 2 ： Use variable character sequences StringBuffer perhaps StringBuilder

Instantiate with constructor StringBuffer perhaps StringBuilder Pay attention to its capacity when , It is recommended to use with parameters new

StringBuffer(int capacity); among StringBuffer Class addition method is append(xxx)

public String replaceSpace1(String s){
        StringBuffer sbuffer = new StringBuffer(s.length() * 3);
        for (char s2 : s.toCharArray()){
            if (s2 == ' ')
                sbuffer.append("%20");
            else
                sbuffer.append(s2);
        }
        String s1 = new String(sbuffer);
        return s1;
    }

Because the function needs to return a string type , There is String and StringBuffer Conversion between , Let's review it here ：

public static void main(String[] args) {
12         //String -> StringBuffer
13         // Create a String object 
14         String str = "Hi Java!";
15         System.out.println(str);
16  
17         // Mode one ： Construction method 
18         StringBuffer buffer = new StringBuffer(str);
19         System.out.println(buffer);
20  
21         // Mode two ： adopt append Method 
22         StringBuffer buffer2 = new StringBuffer();
23         buffer2.append(str);
24         System.out.println(buffer2);
25  
26         //StringBuffer -> String
27         // Create a StringBuffer object 
28         StringBuffer buffer3 = new StringBuffer();
29         buffer3.append("Happy birthday Java!");
30         System.out.println(buffer3);
31  
32         // Mode one ： By construction method 
33         String str2 = new String(buffer3); 
34         System.out.println(str2);
35          
36         // Mode two ： adopt toString Method 
37         String str3 = buffer3.toString();
38         System.out.println(str3);
39     }

Method 3 ： Call directly replace（ Tremble smart ）

public String replaceSpace(String s) { String s1 = s.replace(" ", "%20"); return s1; }

Mutual encouragement ！！