//数组实现String反转
  public String reverseByArray(){
    if(str == null || str.length() == 1){
      return null;
    }
    char[] ch = str.toCharArray();//字符串转换成字符数组
    for(int i = 0 ; i < ch.length/2 ; i++){
      char temp = ch[i];
      ch[i] = ch[ch.length-i-1];
      ch[ch.length-i-1] = temp;
    }
    return new String(ch);
  }

//用栈实现String反转
  public String reverseByStack(){
    if(str == null || str.length() == 1){
      return null;
    }
    Stack<Character> stack = new Stack<Character>();
    char[] ch = str.toCharArray();//字符串转换成字符数组
    for (char c : ch) {
      stack.push(c);//每个字符，推进栈
    }
    for (int i = 0; i < ch.length; i++) {
      ch[i] = stack.pop();//移除这个堆栈的顶部对象
    }
    return new String(ch);
  }

//用逆序遍历实现String反转

  public String reverseBySort(){
    if(str == null || str.length() == 1){
      return null;
    }
    StringBuffer sb = new StringBuffer();
    for (int i = str.length() -1 ; i >= 0; i--) {
      sb.append(str.charAt(i));//使用StringBuffer从右往左拼接字符
    }
    return sb.toString();
  }

//使用位运算实现String反转
  public String reverseByBit() { 
    if(str == null || str.length() == 1){
      return null;
    }
    char[] ch = str.toCharArray();//字符串转换成字符数组
    int len = str.length(); 
    for(int i= 0; i< len/ 2; i++) { 
      ch[i]^= ch[len- 1- i]; 
      ch[len- 1- i]^= ch[i]; 
      ch[i]^= ch[len- 1- i]; 
    }
    return new String(ch);
  }

//使用递归实现String反转
  public String reverseByRecursive(String str){
    if(str == null || str.length() == 0){
      return null;
    }
    if(str.length() == 1){
      return str;
    } else {
      //从下标为1开始截取字符串，在返回下标为0的字符
      return reverseByRecursive(str.substring(1)) + str.charAt(0);
    }
  }

public class Test {
  public static void main(String[] args) {
    String s = "123456";
    Reverse r = new Reverse(s);
    System.out.println(r.reverseByArray());
    System.out.println(r.reverseByStack());
    System.out.println(r.reverseBySort());
    System.out.println(r.reverseByBit());
    System.out.println(r.reverseByRecursive(s));
    
  }
}

public class Reverse {
  private String str = null;
  
  public Reverse(String str){
    this.str = str;
  }
  
  //数组实现String反转
  public String reverseByArray(){
    if(str == null || str.length() == 1){
      return null;
    }
    char[] ch = str.toCharArray();//字符串转换成字符数组
    for(int i = 0 ; i < ch.length/2 ; i++){
      char temp = ch[i];
      ch[i] = ch[ch.length-i-1];
      ch[ch.length-i-1] = temp;
    }
    return new String(ch);
  }
  //用栈实现String反转
  public String reverseByStack(){
    if(str == null || str.length() == 1){
      return null;
    }
    Stack<Character> stack = new Stack<Character>();
    char[] ch = str.toCharArray();//字符串转换成字符数组
    for (char c : ch) {
      stack.push(c);//每个字符，推进栈
    }
    for (int i = 0; i < ch.length; i++) {
      ch[i] = stack.pop();//移除这个堆栈的顶部对象
    }
    return new String(ch);
  }
  //用逆序遍历实现String反转
  public String reverseBySort(){
    if(str == null || str.length() == 1){
      return null;
    }
    StringBuffer sb = new StringBuffer();
    for (int i = str.length() -1 ; i >= 0; i--) {
      sb.append(str.charAt(i));//使用StringBuffer从右往左拼接字符
    }
    return sb.toString();
  }
  //使用位运算实现String反转
  public String reverseByBit() { 
    if(str == null || str.length() == 1){
      return null;
    }
    char[] ch = str.toCharArray();//字符串转换成字符数组
    int len = str.length(); 
    for(int i= 0; i< len/ 2; i++) { 
      ch[i]^= ch[len- 1- i]; 
      ch[len- 1- i]^= ch[i]; 
      ch[i]^= ch[len- 1- i]; 
    }
    return new String(ch);
  }
  //使用递归实现String反转
  public String reverseByRecursive(String str){
    if(str == null || str.length() == 0){
      return null;
    }
    if(str.length() == 1){
      return str;
    } else {
      //从下标为1开始截取字符串，在返回下标为0的字符
      return reverseByRecursive(str.substring(1)) + str.charAt(0);
    }
  }
}