在 java 中使用递归来反转字符串

Question

I want to reverse a whole String . For example, "Cat is running" should give output "running is cat".

I have tried a lot but I am unable to do it. It shows "gninnur si taC". Kindly help me that it should take "cat" as a single character instead of taking 'c' as a single character.

Here is the code:

public static void main(String[] args) {
    String str = "Cat is running";
    System.out.println("Before recursion: " + str);
    System.out.println("After recursion: " + reverse(str));
}

public static String reverse(String str) {
    if(str.isEmpty())
        return str;

    String s = "";
    for(int i = 0; i < str.length(); i++) {
        s = s + str.charAt(i);
    }
    return reverse(s.substring(1)) + s.charAt(0);   
}

Answer 1

You have to find the first word in the String , pass the rest of the String to the recursive call, and append the first word at the end:

public static String reverse(String str) {
    if(str.isEmpty() || !str.contains(" "))
        return str;

    int sep = str.indexOf(' ');
    return reverse(str.substring(sep+1)) + " " + str.substring(0,sep);   
}

Output:

Before recursion: Cat is running
After recursion: running is Cat

BTW, the loop is your original code is pointless. You can simply use str directly instead of creating a copy of it.

You can make it even shorter with:

public static String reverse(String str) {
    int sep = str.indexOf(' ');
    return sep >= 0 ? reverse(str.substring(sep+1)) + " " + str.substring(0,sep) : str;
}

Answer 2

I think I made it a little less good than @Eran but I already wrote:

private static String reverse(String str) {
    if (str.isEmpty() || !str.contains(" "))
        return str;
    StringBuilder sb = new StringBuilder(" ");
    int i = 0;
    while (i < str.length() && str.charAt(i) != ' ') {
        sb.append(str.charAt(i));
        i++;
    }
    return reverse(str.substring(i + 1)) + sb.toString();
}

Answer 3

Here's another option (untested):

public static String reverse(String str) {
  String words[] = str.split (" ");
  StringBuilder sb = new StringBuilder();
  j = words.length - 1;
  while (j >= 0) {
    sb.append(" " + words[j--]);
  }
  return sb.toString();
}