分治问题POJ1854_构造回文

题目

A palindrome is a string of symbols that is equal to itself when reversed. Given an input string, not necessarily a palindrome, compute the number of swaps necessary to transform the string into a palindrome. By swap we mean reversing the order of two adjacent symbols. For example, the string “mamad” may be transformed into the palindrome “madam” with 3 swaps:
swap “ad” to yield “mamda”
swap “md” to yield “madma”
swap “ma” to yield “madam”

Input

The first line of input gives n, the number of test cases. For each test case, one line of input follows, containing a string of up to 8000 lowercase letters.

Output

Output consists of one line per test case. This line will contain the number of swaps, or “Impossible” if it is not possible to transform the input to a palindrome.

Sample Input

3
mamad
asflkj
aabb

Sample Output

3
Impossible
2

import java.util.ArrayList;import java.util.Scanner;/** * Created by 95112 on 10/20/2017. */public class POJA {    public static void main(String[] args)    {        Scanner scanner = new Scanner(System.in);        int n = scanner.nextInt();        ArrayList<String> answers = new ArrayList<>();        for (int i = 0 ; i< n; i++)        {            String input = scanner.next();            int[] alphabet = new int[26];            for (int j = 0; j < 26 ; j++)                alphabet[j] = 0;            for (char tmp: input.toCharArray()){                alphabet[tmp - 'a']++;            }            int count = 0;            for (int tmp : alphabet)                if (tmp%2 == 1)                    count++;            if (count > 1)            {                answers.add("Impossible");                continue;            }            char[] inputChar = input.toCharArray();            int x = 0;            int y = input.length()-1;            int answer = 0;            for ( ; x < input.length() && x < y ; x++,y--){                int left = x;                int right = y;                for (;left < y; left++){                    if (inputChar[left] == inputChar[y])                        break;                }                for (; right > x ; right--){                    if (inputChar[right] == inputChar[x])                        break;                }                if (left - x > (y - right))                {                    char deal;                    answer += (y - right);                    for (int start = right ; start < y; start++ )                    {                        deal = inputChar[start+1];                        inputChar[start+1] = inputChar[start];                        inputChar[start] = deal;                    }                }                else {                    char deal;                    answer += (left-x);                    for (int start = left; start > 0 ; start--)                    {                        deal = inputChar[start];                        inputChar[start] = inputChar[start-1];                        inputChar[start-1] = deal;                    }                }            }            answers.add(String.valueOf(answer));        }        for (String answer : answers)            System.out.println(answer);    }}