LeetCode – Distinct Subsequences Total (Java)

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Given a string S and a string T, count the number of distinct subsequences of T in S.

A subsequence of a string is a new string which is formed from the original string by deleting some (can be none) of the characters without disturbing the relative positions of the remaining characters. (ie, “ACE” is a subsequence of “ABCDE” while “AEC” is not).

Here is an example:
S = “rabbbit”, T = “rabbit”

Return 3.

Analysis

The problem itself is very difficult to understand. It can be stated like this:
Give a sequence S and T, how many distinct sub sequences from S equals to T?
How do you define “distinct” subsequence? Clearly, the ‘distinct’ here mean different operation combination, not the final string of subsequence. Otherwise, the result is always 0 or 1. — from Jason’s comment

When you see string problem that is about subsequence or matching, dynamic programming method should come to mind naturally. The key is to find the initial and changing condition.

Java Solution 1

Let W(i, j) stand for the number of subsequences of S(0, i) equals to T(0, j). If S.charAt(i) == T.charAt(j), W(i, j) = W(i-1, j-1) + W(i-1,j); Otherwise, W(i, j) = W(i-1,j).

public int numDistincts(String S, String T) {
	int[][] table = new int[S.length() + 1][T.length() + 1];
 
	for (int i = 0; i < S.length(); i++)
		table[i][0] = 1;
 
	for (int i = 1; i <= S.length(); i++) {
		for (int j = 1; j <= T.length(); j++) {
			if (S.charAt(i - 1) == T.charAt(j - 1)) {
				table[i][j] += table[i - 1][j] + table[i - 1][j - 1];
			} else {
				table[i][j] += table[i - 1][j];
			}
		}
	}
 
	return table[S.length()][T.length()];
}

Java Solution 2

Do NOT write something like this, even it can also pass the online judge.

public int numDistinct(String S, String T) {
	HashMap<Character, ArrayList<Integer>> map = new HashMap<Character, ArrayList<Integer>>();
 
	for (int i = 0; i < T.length(); i++) {
		if (map.containsKey(T.charAt(i))) {
			map.get(T.charAt(i)).add(i);
		} else {
			ArrayList<Integer> temp = new ArrayList<Integer>();
			temp.add(i);
			map.put(T.charAt(i), temp);
		}
	}
 
	int[] result = new int[T.length() + 1];
	result[0] = 1;
 
	for (int i = 0; i < S.length(); i++) {
		char c = S.charAt(i);
 
		if (map.containsKey(c)) {
			ArrayList<Integer> temp = map.get(c);
			int[] old = new int[temp.size()];
 
			for (int j = 0; j < temp.size(); j++)
				old[j] = result[temp.get(j)];
 
			// the relation
			for (int j = 0; j < temp.size(); j++)
				result[temp.get(j) + 1] = result[temp.get(j) + 1] + old[j];
		}
	}
 
	return result[T.length()];
}

8 thoughts on “LeetCode – Distinct Subsequences Total (Java)”

  1. Agree, I had to look up in leetcode for a better description (https://leetcode.com/problems/distinct-subsequences/):
    “Given two strings s and t, return the number of distinct subsequences of s which equals t”
    The example is:

    Input: s = “rabbbit”, t = “rabbit”
    Output: 3
    Explanation:
    As shown below, there are 3 ways you can generate “rabbit” from S.
    rabbbit
    rabbbit
    rabbbit

  3. T itself is the possible subsequence of S. In this example T = “rabbit” and S = “rabbbit” so your looking for sequences of “rabbit” within “rabbbit”. Possible subsequences are:
    [r] [a] [b] [b] b [i] [t]
    [r] [a] [b] b [b] [i] [t]
    [r] [a] b [b] [b] [i] [t]

  4. Shouldn’t there be more subsequences in the above example ? Shouldn’t ra, rab, ab be counted as well ?

  5. I was wondering how shall we decide whether source string should be considered as a row or source string should be considered as a column.

  7. O(n) space solution:

    public class Solution {
    public int numDistinct(String S, String T) {
    if (S == null || T == null || S.length() == 0 || T.length() == 0
    || S.length() < T.length()) {
    return 0;
    }

    char[] s = S.toCharArray();
    char[] t = T.toCharArray();

    int[] a = new int[s.length];
    if (s[0] == t[0]) {
    a[0] = 1;
    }

    for (int i = 1; i < s.length; ++i) {
    if (s[i] == t[0]) {
    a[i] = a[i – 1] + 1;
    } else {
    a[i] = a[i – 1];
    }
    }

    for (int i = 1; i < t.length; ++i) {
    int prev = a[i – 1];
    a[i – 1] = 0;
    for (int j = i; j < s.length; ++j) {
    int next = a[j];
    if (s[j] == t[i]) {
    a[j] = prev + a[j – 1];
    } else {
    a[j] = a[j – 1];
    }
    prev = next;
    }
    }

    return a[a.length – 1];
    }
    }

  8. The problem itself is very difficult to understand. It can be stated like this:
    Give a sequence S and T, how many distinct sub sequences from S equals to T?
    How do you define “distinct” subsequence? Clearly, the ‘distinct’ here mean different operation combination, not the final string of subsequence. Otherwise, the result is always 0 or 1. But isn’t every sub sequence from a distinct operation? So does distinct have any practical meaning here?

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