163. Distinct Subsequences

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Example 1:

Input: s = "rabbbit", t = "rabbit"

Output: 3

Explanation: There are 3 distinct subsequences of "rabbbit" that equal "rabbit": rabbbit, rabbbit, rabbbit.

Example 2:

Input: s = "babgbag", t = "bag"

Output: 5

Explanation: There are 5 distinct subsequences of "babgbag" that equal "bag": babgbag, babgbag, babgbag, babgbag, babgbag.

Example 3:

Input: s = "", t = ""

Output: 1

Explanation: An empty string has one subsequence that is an empty string.

Example 4:

Input: s = "abc", t = ""

Output: 1

Explanation: The empty string is always a subsequence of any string, so there is one way.

Example 5:

Input: s = "", t = "abc"

Output: 0

Explanation: If s is empty but t is not, then t cannot be a subsequence of s.

Constraints:

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Time Complexity

O(m × n)

Space Complexity