131. Palindrome Partitioning

/*
 * 131. Palindrome Partitioning 
My Submissions QuestionEditorial Solution
Total Accepted: 66490 Total Submissions: 239524 Difficulty: Medium
Given a string s, partition s such that every substring of the partition is a palindrome.

Return all possible palindrome partitioning of s.

For example, given s = "aab",
Return

[
  ["aa","b"],
  ["a","a","b"]
]
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 */
package dfs;

import java.util.*;

public class ParlindromePartition {

    public static void main(String[] args) {
        // TODO Auto-generated method stub
        ParlindromePartition pt = new ParlindromePartition();
        String s = "aab";
        List> res = pt.partition(s);
        // System.out.print(res);

    }

    public List> partition(String s) {
        /*
         * Time complexity is O(n*(2^n)). The function isPalindrome is O(n)
         * https://leetcode.com/discuss/18984/java-backtracking-solution if the
         * input is "aab", check if [0,0] "a" is palindrome. then check [0,1]
         * "aa", then [0,2] "aab".
         * 
         * While checking [0,0], the rest of string is "ab", use ab as input to
         * make a recursive call.
         */
        List> res = new ArrayList>();
        List cur = new ArrayList<>();

        backtrack(res, cur, 0, s);
        System.out.print(res);
        return res;
    }

    public void backtrack(List> res, List cur, int pos, String s) {
        if (pos == s.length()) {
            res.add(new ArrayList(cur));
        }

        for (int i = pos; i < s.length(); i++) {
            if (isPalindrome(s, pos, i)) {
                cur.add(s.substring(pos, i + 1));
                backtrack(res, new ArrayList(cur), i + 1, s);
                cur.remove(cur.size() - 1);
            }
        }
    }

    public boolean isPalindrome(String str, int l, int r) {
        while (l < r) {
            if (str.charAt(l++) != str.charAt(r--)) {
                return false;
            }
        }
        return true;
    }

}