Package g1901_2000.s1955_count_number_of_special_subsequences

Class Solution

  • All Implemented Interfaces:
    
public final class Solution

    1955 - Count Number of Special Subsequences\.

    Hard

    A sequence is special if it consists of a positive number of 0s, followed by a positive number of 1s, then a positive number of 2s.

    • For example, [0,1,2] and [0,0,1,1,1,2] are special.

    • In contrast, [2,1,0], [1], and [0,1,2,0] are not special.

    Given an array nums (consisting of only integers 0, 1, and 2), return the number of different subsequences that are special. Since the answer may be very large, return it modulo <code>10<sup>9</sup> + 7</code>.

    A subsequence of an array is a sequence that can be derived from the array by deleting some or no elements without changing the order of the remaining elements. Two subsequences are different if the set of indices chosen are different.

    Example 1:

    Input: nums = 0,1,2,2

    Output: 3

    Explanation: The special subsequences are bolded **0** , **1** , **2** ,2, **0** , **1** ,2, **2** , and **0** , **1** , **2** , **2** .

    Example 2:

    Input: nums = 2,2,0,0

    Output: 0

    Explanation: There are no special subsequences in 2,2,0,0.

    Example 3:

    Input: nums = 0,1,2,0,1,2

    Output: 7

    Explanation: The special subsequences are bolded:

    • **0** , **1** , **2** ,0,1,2

    • **0** , **1** ,2,0,1, **2**

    • **0** , **1** , **2** ,0,1, **2**

    • **0** , **1** ,2,0, **1** , **2**

    • **0** ,1,2, **0** , **1** , **2**

    • **0** ,1,2,0, **1** , **2**

    • 0,1,2, **0** , **1** , **2**

    Constraints:

    • <code>1 <= nums.length <= 10<sup>5</sup></code>

    • 0 &lt;= nums[i] &lt;= 2

    • Nested Class Summary

      Nested Classes 
      Modifier and Type Class Description

    • Field Summary

      Fields 
      Modifier and Type Field Description

    • Constructor Summary

      Constructors 
      Constructor Description
      Solution()

    • Enum Constant Summary

      Enum Constants 
      Enum Constant Description

    • Method Summary

      Modifier and Type Method Description
      final Integer countSpecialSubsequences(IntArray nums)

      • Methods inherited from class java.lang.Object

        clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait