Palindrome Pairs

Given a list of unique words. Find all pairs of distinct indices (i, j) in the given list, so that the concatenation of the two words, i.e. words[i] + words[j] is a palindrome.

Example 1:
Given words = ["bat", "tab", "cat"]
Return [[0, 1], [1, 0]]
The palindromes are ["battab", "tabbat"]

Example 2:
Given words = ["abcd", "dcba", "lls", "s", "sssll"]
Return [[0, 1], [1, 0], [3, 2], [2, 4]]
The palindromes are ["dcbaabcd", "abcddcba", "slls", "llssssll"]

方法一：暴力解法

public class PalindromePairs {
    public boolean isPalindrome(String string) {
        char c[]=string.toCharArray();
        boolean flag=true;
        for (int i = 0; i < c.length/2; i++) {
            if (c[i]!=c[c.length-i-1]) {
                flag=false;
                break;
            }
        }
        return flag;
    }
    public List<List<Integer>> palindromePairs(String[] words) {
        List<List<Integer>> b= new ArrayList<List<Integer>>();
        for (int i = 0; i < words.length; i++) {
            for (int j = 0; j < words.length; j++) {
                if (i!=j) {
                    String string=words[i]+words[j];
                    if (isPalindrome(string)) {
                        List<Integer> a= new ArrayList<Integer>();
                        a.add(i);
                        a.add(j);
                        b.add(a);
                    }
                }
            }
        }
        return b;
    }
    public static void main(String[] args) {
        String [] words={"hijajcd","hjhgahbcegj"};
        PalindromePairs pairs=new PalindromePairs();
        System.out.println(pairs.palindromePairs(words));
    }
}

方法二：

利用

public class Solution {
    public List<List<Integer>> palindromePairs(String[] words) {
        List<List<Integer>> resultList= new ArrayList<List<Integer>>();
        Map<String,Integer> map = new HashMap<String, Integer>();
        for (int i = 0; i < words.length; i++) {
            map.put(words[i], i);
        }
        for (int i = 0; i < words.length; i++) {
            for (int j = 0; j <= words[i].length(); j++) {
                String subString=words[i].substring(0,j);
                String subString2=words[i].substring(j);
                if (isPalindrome(subString)) {
                    if (map.get(reverseString(subString2))!=null) {
                        ArrayList<Integer> arrayList=new ArrayList<Integer>(2);
                        arrayList.add(map.get(reverseString(subString2)));
                        arrayList.add(map.get(words[i]));
                        if (!arrayList.get(0).equals(arrayList.get(1))) {
                            resultList.add(arrayList);
                        }
                    }
                }
                if (isPalindrome(subString2)) {
                    if (map.get(reverseString(subString))!=null) {
                        ArrayList<Integer> arrayList=new ArrayList<Integer>(2);
                        arrayList.add(map.get(words[i]));
                        arrayList.add(map.get(reverseString(subString)));
                        if (!arrayList.get(0).equals(arrayList.get(1))) {
                            resultList.add(arrayList);
                        }
                    }
                }
            }
        }
        HashSet hashSet=new HashSet(resultList);
        resultList.clear();
        resultList.addAll(hashSet);
        return resultList;

    }
        public boolean isPalindrome(String string) {
        char c[]=string.toCharArray();
        boolean flag=true;
        for (int i = 0; i < c.length/2; i++) {
            if (c[i]!=c[c.length-i-1]) {
                flag=false;
                break;
            }
        }
        return flag;
    }

        public String reverseString(String string) {
        char[] chars=string.toCharArray();
        char[] result=new char[chars.length];
        for (int i = 0; i < chars.length; i++) {
            result[chars.length-i-1]=chars[i];
        }
        return new String(result);
    }
}