g1901_2000.s1955_count_number_of_special_subsequences

Class Solution

java.lang.Object

g1901_2000.s1955_count_number_of_special_subsequences.Solution

public class Solution
extends Object

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 109 + 7.

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:

• 1 <= nums.length <= 105
• 0 <= nums[i] <= 2

Constructor Summary

Constructors

Constructor and Description
Solution()

Method Summary

Modifier and Type	Method and Description
int	countSpecialSubsequences(int[] nums)

Methods inherited from class java.lang.Object

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

Constructor Detail

Solution

public Solution()

Method Detail

countSpecialSubsequences

public int countSpecialSubsequences(int[] nums)