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How to Solve Count Number of Special Subsequences Problem
Master the Count Number of Special Subsequences LeetCode problem with undetectable real-time assistance. Get instant solutions and explanations during your coding interviews.
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LeetCode ProblemCount Number of Special Subsequences
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. 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.
ArrayDynamic Programming
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Problem Breakdown
Understanding the Count Number of Special Subsequences Problem
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Problem Statement
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. 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.
HardProblem #1955
LeetCodeCount Number of Special Subsequences
Related Topics
ArrayDynamic Programming
How Phantom Code Helps
Get real-time assistance for Count Number of Special Subsequences problems during coding interviews. Phantom Code provides instant solutions and explanations.
Examples
# Example 1
Input
nums = [0,1,2,2]
Output
3
# Example 2
Input
nums = [2,2,0,0]
Output
0
# Example 3
Input
nums = [0,1,2,0,1,2]
Output
7
Constraints
1 <= nums.length <= 105
0 <= nums[i] <= 2
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Solve Count Number of Special Subsequences — A sequence is special if it consists of a positive number of 0s, followed by a p...
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Frequently Asked Questions
Common questions about solving Count Number of Special Subsequences and using Phantom Code during coding interviews.
How does Phantom Code help me solve coding problems like Count Number of Special Subsequences during interviews?
Phantom Code generates complete solutions instantly with proper complexity analysis, letting you focus on explaining your approach and demonstrating problem-solving skills rather than getting stuck on implementation details during high-pressure situations.
What if the interviewer asks follow-up questions or modifications to problems like Count Number of Special Subsequences?
Phantom Code adapts in real-time. Screenshot the modified problem or use audio mode to capture the interviewer's follow-up, and get an updated solution within seconds — including edge cases and optimizations.
Can Phantom Code help me communicate my solution for problems like Count Number of Special Subsequences?
Yes. Phantom Code provides step-by-step approach explanations with 3 progressive thoughts, so you can walk through your solution naturally. It also provides time and space complexity analysis to discuss trade-offs.
How does Phantom Code assist with different programming languages for Count Number of Special Subsequences type problems?
Phantom Code supports 11 programming languages including Python, Java, C++, JavaScript, TypeScript, Go, Rust, Ruby, Swift, Kotlin, and C#. Set your preferred language and get idiomatic solutions.
What coding mistakes does Phantom Code help me avoid during Count Number of Special Subsequences interviews?
Phantom Code catches common mistakes like off-by-one errors, missing edge cases, incorrect base conditions, and suboptimal approaches. It provides clean, well-structured code that handles all edge cases.
Is Phantom Code detectable when solving problems like Count Number of Special Subsequences during live interviews?
No. Phantom Code runs as an invisible desktop overlay that doesn't appear in screen shares, recordings, or proctoring software. It works with Zoom, HackerRank, CodeSignal, CoderPad, and all major platforms.
How does Phantom Code help when I'm struggling with Count Number of Special Subsequences style questions under pressure?
The AI provides structured guidance: first the approach (what algorithm/data structure to use and why), then the implementation with clean code, and finally complexity analysis. This helps you think clearly even under pressure.
Does Phantom Code work for the coding platforms used in Count Number of Special Subsequences interviews?
Yes. Phantom Code works with all major coding platforms including LeetCode, HackerRank, CodeSignal, CoderPad, HackerEarth, and any browser-based coding environment.
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