字符串的反转

jdk的方法：

String str = “abcdef”; 
str = new StringBuilder(str).reverse().toString();

自己实现：

public class StringDemo{
    public static void main(String[] args) {
        System.out.println(reverse("Hello World!"));//!dlroW olleH
    }
    public static String reverse(String str){
       char [] arrs = str.toCharArray();
        char temp = ' ';
        for(int i = 0, j = arrs.length-1; i < j; i++,j--){
            temp = arrs[i];
           arrs[i] = arrs[j];
           arrs[j] = temp;
        }
        StringBuilder sb = new StringBuilder();
        for(char s : arrs){
           sb.append(s);
        }
        return sb.toString();
    }
}

public void reverse2(final String str){
   char [] chars = str.toCharArray();
   StringBuilder sb = new StringBuilder();
   for(int i = chars.length-1;i>0; i--){
      sb.append(char[i]);
   }
   return sb.toString();
}

* “I love china”, 把字符反转后变成 “china love I”
方法一：一个字符一个字符的换

public class StringReverse {
    
    public void swap(char[] arr, int begin, int end) {
        while(begin < end) {
            char temp = arr[begin];
            arr[begin] = arr[end];
            arr[end] = temp;
            begin++;
            end--;
        }
    }
    public String swapWords(String str) {
        char[] arr = str.toCharArray();
        swap(arr, 0, arr.length - 1);
        int begin = 0;
        for (int i = 1; i < arr.length; i++) {
            if (arr[i] == ' ') {
                swap(arr, begin, i - 1);
                begin = i + 1;
            }
        }

        return new String(arr);
    }
    public static void main(String[] args) {
        String str = "I love java";
        System.out.println(new StringReverse().swapWords(str));
    }
}

方法二：（利用jdk提供的方法）

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        String str = sc.nextLine();
        String[] sArr = str.split(" ");//I love java
        List<String> list = Arrays.asList(sArr);
        Collections.reverse(list);
        for(String word:list){
            System.out.print(word+" ");
        }
    }
}

方法3：一个单词一个单词地换

public class StringDemo{
    public static void main(String[] args) {
        System.out.println(reverse("Hello World!"));//World! Hello
    }
    public static String reverse(String str){
    String [] arrs = str.split(" ");
        String temp = "";
        for(int i = 0, j = arrs.length-1; i < j; i++, j--){
            temp = arrs[i];
           arrs[i] = arrs[j];
           arrs[j] = temp;
        }
        StringBuilder sb = new StringBuilder();
        for(String s : arrs){
           sb.append(s).append(" ");
        }
      return sb.toString();
    }
}