如何在Java中将给定字符串中的字符减少到回文

Question

Here i have code to check whether given string is palindrome or not but don't know the procedure to reduce characters from last.To make change the character i need to follow the rule as: Eg String input="abcde" .Letter 'e' can be converted to 'd' but not 'e' to 'd' if character becomes 'a' we can't change further.Here is my code:

 public static void main(String[] args) {
    String input = "abcd";
    String temp = input;
    String output = "";
    for (int i = output.length() - 1; i >= 0; i--) {
        output = output + output.charAt(i);
    }
    if(temp.equals(output)){
        System.out.println("String is palindrome");
    }else{
        System.out.println("Not a palindrome");
    }
}

output order logic is as follows:

if input is =abcd
    abcc('d' converted to 'c')and check for palindrome
    abcb('c' converted to 'b')and check for palindrome
    abca('b' converted to 'a')and check for palindrome
    abba('a' further we can't change so shift to previous letter 'c' and change to 'b') 
          and check for palindrome

String is palindrome and count is=4

if input=cbdf
    cbde('f' converted to 'f')and check for palindrome
    cdbd('e' converted to 'd')and check for palindrome
    cdbc('d' converted to 'c')and check for palindrome
    cdbb('c' converted to 'b')and check for palindrome
    cdba('b' converted to 'a')and check for palindrome
    cdaa('a' further we can't change so shift to previous letter 'b' and change to 'a')
      and check for palindrome
    ccaa('a' further we can't change so shift to previous letter 'd' and change to 'c')
      and check for palindrome
    cbaa('c' converted to 'b')and check for palindrome
    caaa('a' further we can't change so shift to previous letter 'c' and change to 'b')
       and check for palindrome
    baaa('b' converted to 'a')and check for palindrome
    aaaa now string is palindrome
    String is palindrome and count is=10
   finally i need the count to make string palindrome.

Answer 1

Assuming that I understand your problem, you are going about this in an inefficent way. Here is how I would do this:

String toPalindrome(String str){
    StringBuilder reverse = new StringBuilder(str).reverse();

    for(int idx = 0; idx < str.size()/2; idx++)
        if(str.getCharAt(idx) < reverse.getCharAt(idx))
            reverse.setCharAt(idx, str.getCharAt(idx));

    return reverse.subString(0,str.size()/2) + reverse.reverse().subString(str.size()/2);
}

Aside from the off-by-one errors in this code, this should work to produce the output you need.

Using this approach, we don't have to decrement each character one by one - we rather just immediately replace each character with the target value. We also never have to check if it is a palindrome, since this method is guaranteed to produce one (after all, it is concatenating a string with its mirror image in the return step).

EDIT: seeing that we need only return the number of times characters were decremented, we can do something even simpler:

int abs(int x){
    return x>0?x:-x;
}

int palindromeCounts(String str){
    int count = 0;

    for(int idx = 0; idx < str.length()/2; idx++)
        count += abs(str.charAt(idx) - reverse.charAt(str.length()-1-idx));

    return count;
}