How do you solve Number of Times Binary String Is Prefix-Aligned on LeetCode?

Number of Times Binary String Is Prefix-Aligned (LeetCode #1375) can be solved using Brute Force or Optimal Solution. The optimal approach is Optimal Solution with O(n) time complexity and O(1) space complexity. Key insight: The maximum index flipped must equal the current step index for the prefix to be aligned.

What is the time complexity of Number of Times Binary String Is Prefix-Aligned?

The optimal solution for Number of Times Binary String Is Prefix-Aligned runs in O(n) time and uses O(1) space. The time complexity is O(n) because we only iterate through the flips array once, and we use constant space for our variables.

What is the brute force solution for Number of Times Binary String Is Prefix-Aligned?

The brute force approach for Number of Times Binary String Is Prefix-Aligned is Brute Force, with O(n²) time complexity and O(1) space complexity. The brute force approach involves simulating the flipping of each bit and checking if the prefix is aligned after each flip. This means we will check the entire prefix each time to see if all bits are 1's. The optimal Optimal Solution approach improves this to O(n).

What algorithm or data structure is used to solve Number of Times Binary String Is Prefix-Aligned?

Number of Times Binary String Is Prefix-Aligned uses the following concepts: Array. The recommended patterns to study are: Array, Greedy.

What are the interview tips for Number of Times Binary String Is Prefix-Aligned?

Always clarify whether the input is 0-indexed or 1-indexed. Think about how to reduce the number of checks needed for conditions like alignment. Practice identifying patterns in problems, such as tracking maximums or using counters.

What are common mistakes when solving Number of Times Binary String Is Prefix-Aligned?

Students often forget to account for 1-indexing in the flips array. Some may try to simulate the entire binary string instead of focusing on the indices.

#1375

Number of Times Binary String Is Prefix-Aligned

Medium

ArrayArrayGreedy

LeetCode ↗

Approaches

Brute ForceOptimal

Complexity Comparison

	Brute Force	Optimal Solution★
Time	O(n²)	O(n)
Space	O(1)	O(1)

💡

Intuition

Time O(n)Space O(1)

Instead of checking the prefix after each flip, we can keep track of the maximum index flipped so far and compare it to the current step. If they match, it means all previous bits are aligned.

⚙️

Algorithm

3 steps

1Step 1: Initialize a variable to keep track of the maximum index flipped so far.
2Step 2: For each flip in the flips array, update the maximum index flipped.
3Step 3: If the maximum index flipped equals the current step index (1-indexed), increment the count of prefix-aligned occurrences.

solution.py8 lines

1def countPrefixAligned(flips):
2    max_flipped = 0
3    count = 0
4    for i in range(len(flips)):
5        max_flipped = max(max_flipped, flips[i])
6        if max_flipped == i + 1:
7            count += 1
8    return count

ℹ

Complexity note: The time complexity is O(n) because we only iterate through the flips array once, and we use constant space for our variables.

1The maximum index flipped must equal the current step index for the prefix to be aligned.
2We can optimize the check for alignment by tracking the maximum index flipped instead of checking the entire prefix.

Solutions and explanations are original Tejav content. Problem titles © LeetCode — use the LeetCode button above for the full problem statement.