LeetCode – Palindrome Partitioning (Java)

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Problem

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"]
  ]

1. Depth-first Search

public ArrayList<ArrayList<String>> partition(String s) {
	ArrayList<ArrayList<String>> result = new ArrayList<ArrayList<String>>();
 
	if (s == null || s.length() == 0) {
		return result;
	}
 
	ArrayList<String> partition = new ArrayList<String>();//track each possible partition
	addPalindrome(s, 0, partition, result);
 
	return result;
}
 
private void addPalindrome(String s, int start, ArrayList<String> partition,
		ArrayList<ArrayList<String>> result) {
	//stop condition
	if (start == s.length()) {
		ArrayList<String> temp = new ArrayList<String>(partition);
		result.add(temp);
		return;
	}
 
	for (int i = start + 1; i <= s.length(); i++) {
		String str = s.substring(start, i);
		if (isPalindrome(str)) {
			partition.add(str); 
			addPalindrome(s, i, partition, result);
			partition.remove(partition.size() - 1);
		}
	}
}
 
private boolean isPalindrome(String str) {
	int left = 0;
	int right = str.length() - 1;
 
	while (left < right) {
		if (str.charAt(left) != str.charAt(right)) {
			return false;
		}
 
		left++;
		right--;
	}
 
	return true;
}

2. Dynamic Programming

The dynamic programming approach is very similar to the problem of longest palindrome substring.

public static List<String> palindromePartitioning(String s) {
 
	List<String> result = new ArrayList<String>();
 
	if (s == null)
		return result;
 
	if (s.length() <= 1) {
		result.add(s);
		return result;
	}
 
	int length = s.length();
 
	int[][] table = new int[length][length];
 
	// l is length, i is index of left boundary, j is index of right boundary
	for (int l = 1; l <= length; l++) {
		for (int i = 0; i <= length - l; i++) {
			int j = i + l - 1;
			if (s.charAt(i) == s.charAt(j)) {
				if (l == 1 || l == 2) {
					table[i][j] = 1;
				} else {
					table[i][j] = table[i + 1][j - 1];
				}
				if (table[i][j] == 1) {
					result.add(s.substring(i, j + 1));
				}
			} else {
				table[i][j] = 0;
			}
		}
	}
 
	return result;
}

11 thoughts on “LeetCode – Palindrome Partitioning (Java)”

  1. Just some shorter version for recursive solution:

    public static void palindrome(String input, List temp, List<List> result) {
    for (int i = 1; i <= input.length(); i++) {
    String str = input.substring(0, i);
    if (isPalindrome(str)) {
    List l = new ArrayList(temp);
    l.add(str);
    if (!input.substring(i, input.length()).isEmpty()) {
    palindrome(input.substring(i, input.length()), l, result);
    } else {
    result.add(l);
    }
    }
    }
    }

    public static void main(String[] args) {
    String input = “aab”;
    List<List> result = new ArrayList();
    palindrome(input, new ArrayList(), result);
    System.out.println(result);
    }

  4. The Dynamic programming solution really just gives all possible palindromes. It doesn’t really partition the string in various ways,right?

  7. Using dynamic method of complexity O(n*n)

    public static List palindromePartitioning(String s) {

    List result = new ArrayList();

    if (s == null)
    return result;

    if(s.length() <=1) {
    result.add(s);
    return result;
    }

    int length = s.length();

    int[][] table = new int[length][length];

    //condition for calculate whole table
    for (int l = 1; l <= length; l++) {
    for (int i = 0; i <= length-l; i++) {
    int j = i + l – 1;
    if (s.charAt(i) == s.charAt(j)) {
    if(l == 1 || l == 2) {
    table[i][j] = 1;
    } else {
    table[i][j] = table[i + 1][j – 1];
    }
    if (table[i][j] == 1) {
    result.add(s.substring(i, j + 1));
    }
    } else {
    table[i][j] = 0;
    }
    }
    }

    return result;
    }

  10. Nope, you cannot. Remember Java does pass by reference. So if you dont assign that temp to a new instance, all other temps will be changed.

  11. Why do you assign partition to temp in the stop condition? Can you add partition to the result directly?

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