leetcode@ [336] Palindrome Pairs (HashMap)

https://leetcode.com/problems/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"]

public class Solution {

public static boolean isPalindrome(StringBuffer sb) {

if(sb == null || sb.length() == 0)  return true;

int l = 0, r = sb.length()-1;

while(l < r) {
if(sb.charAt(l) != sb.charAt(r))  return false;
++l;  --r;
}

return true;
}

public static void buildMappings(String[] words, HashMap<String, HashSet<String> >  mapping) {

for(int i=0; i<words.length; ++i) {

StringBuffer sb = new StringBuffer(words[i]);
HashSet<String> adj = new HashSet<String> ();

for(int j=0; j<=sb.length(); ++j) {
/** partition words[i] into two parts */
StringBuffer prefix = new StringBuffer(sb.substring(0, j));
StringBuffer suffix = new StringBuffer(sb.substring(j));

/** check whether or not the prefix of words[i] is palindrome */
/** if it does, then the prefix can be the rotated point, otherwise not*/
if(isPalindrome(prefix)) {
/** adj stores the strings where suffix + words[i] is palindrome */
adj.add(suffix.reverse().toString());
}

if(isPalindrome(suffix)) {
/** adj stores the strings where words[i] + prefix is palindrome */
adj.add(prefix.reverse().toString());
}
}
/** add it to mapping */
mapping.put(sb.toString(), adj);

}

HashSet<String> hs = new HashSet<String> ();
for(int i=0; i<words.length; ++i) {

StringBuffer sb = new StringBuffer(words[i]);

if(isPalindrome(sb)) {
hs.add(sb.toString());
}
}

for(int i=0; i<words.length; ++i) {

if(words[i].equals("")) {
HashSet<String> adj = new HashSet<String> ();
adj.addAll(hs);
mapping.put(words[i], adj);
}
}

}

public List<List<Integer>> palindromePairs(String[] words) {

HashSet<List<Integer> > hres = new HashSet<List<Integer> > ();
List<List<Integer> > res = new ArrayList<List<Integer> > ();

HashMap<String, HashSet<String> >  mapping = new HashMap<String, HashSet<String> > ();
buildMappings(words, mapping);

HashMap<String, Integer> dict = new HashMap<String, Integer> ();
for(int i=0; i<words.length; ++i) {
dict.put(words[i], i);
}

Iterator iter = mapping.entrySet().iterator();
while(iter.hasNext()) {
HashMap.Entry entry = (HashMap.Entry) iter.next();

String str = (String) entry.getKey();
int lpos = dict.get(str);
//System.out.print(str + " ==> ");
HashSet<String> hs = (HashSet<String>) entry.getValue();
Iterator<String> itc = hs.iterator();

while(itc.hasNext()) {
String s = itc.next();

//System.out.print(s + "  ");

if(dict.containsKey(s)) {
int rpos = dict.get(s);
if(lpos != rpos) {

StringBuffer sb1 = new StringBuffer(str).append(s);
if(isPalindrome(sb1)) {
List<Integer> ll = new ArrayList<Integer> ();
ll.add(lpos);
ll.add(rpos);
hres.add(ll);
}

StringBuffer sb2 = new StringBuffer(s).append(str);
if(isPalindrome(sb2)) {
List<Integer> ll2 = new ArrayList<Integer> ();
ll2.add(rpos);
ll2.add(lpos);
hres.add(ll2);
}
}
}

}//System.out.println();
}

res.addAll(hres);

return res;
}
}

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