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Maximum Number of Operations to Move Ones to the End - Solution & Explanation

MediumStringGreedyCounting15 min readAsked at: Amazon, Meta, Google +1
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Problem Statement

You are given a binary string s.

You can perform the following operation on the string any number of times:

  • Choose any index i from the string where i + 1 < s.length such that s[i] == '1' and s[i + 1] == '0'.
  • Move the character s[i] to the right until it reaches the end of the string or another '1'. For example, for s = "010010", if we choose i = 1, the resulting string will be s = "000110".

Return the maximum number of operations that you can perform.

 

Example 1:

Input: s = "1001101"

Output: 4

Explanation:

We can perform the following operations:

  • Choose index i = 0. The resulting string is s = "0011101".
  • Choose index i = 4. The resulting string is s = "0011011".
  • Choose index i = 3. The resulting string is s = "0010111".
  • Choose index i = 2. The resulting string is s = "0001111".

Example 2:

Input: s = "00111"

Output: 0

 

Constraints:

  • 1 <= s.length <= 105
  • s[i] is either '0' or '1'.

Approach Overview

Problem Overview: Given a binary string s, determine the maximum number of operations required to move all 1s to the end. An operation effectively moves a 1 to the right across a 0. The goal is to count how many such moves can occur before all 1s are positioned after every 0.

Approach 1: Greedy Counting (O(n) time, O(1) space)

The key observation: every time a 1 appears before a 0, that pair will eventually require a move. This turns the problem into counting inversions of the form (1,0). Scan the string left to right while tracking how many 1s have been seen. When a 0 appears, all previously seen 1s could move past it through operations, so add that count to the result. Continue until the end of the string. This greedy scan works because each 1 can cross each 0 at most once, and counting these pairs directly gives the maximum number of operations.

This technique is common in problems involving binary ordering and swap-style operations, especially in greedy and string processing tasks. The algorithm performs a single pass and uses only a couple of counters.

Approach 2: Scan and Balance Counting (O(n) time, O(1) space)

Another way to view the problem is by balancing the number of 1s that still need to move. Iterate through the string while maintaining a running count of active 1s. Each time a 0 appears and there are pending 1s to the left, those 1s will eventually move across this position. Add the active 1 count to the total operations. Conceptually, the algorithm treats each 0 as a checkpoint that all previous 1s must cross.

This formulation highlights the counting aspect of the problem and mirrors how many swap-like transformations occur when pushing elements to the end. Implementation remains a single linear scan with constant memory.

Recommended for interviews: The greedy counting approach. It reduces the problem to counting 1-before-0 pairs in one pass, which demonstrates strong pattern recognition. Explaining the inversion-style insight shows the interviewer you can convert a simulation problem into a simple counting strategy.

Approach 1: Greedy Approach

In this approach, we will iterate through the string and count the number of operations we can perform. Each valid operation will involve swapping a pair of '10' to '01' by tracking the zeros followed by ones.

The function maximumOperations iterates through the string, counting the number of zeros we encounter. When a '1' follows the counted zeros, we can perform operations to push this '1' through all zero positions, incrementing our operations counter. This allows all '1's to continually shift through to the end.

Code

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Complexity

Time Complexity: O(n), as we iterate through the string once.
Space Complexity: O(1), using constant extra space.

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Approach 2: Scan and Balance Counting

In this approach, we track the imbalance between '1's and '0's to calculate possible operations. The idea is to find how many '1's can become "stuck" behind '0's and count how many operations it would take to move each '1' to the end of the '0s'.

Here, the string is scanned to calculate an imbalance, which increases when a '1' is encountered and contributes to potential operations whenever a '0' follows. This approach tallies up by simulating moving '1's over '0's, counting each necessary move.

Code

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Complexity

Time Complexity: O(n), going through the string once.
Space Complexity: O(1), no extra space beyond constant counters.

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Approach 3: Greedy

We use a variable ans to record the answer and another variable cnt to count the current number of 1s.

Then, we iterate through the string s. If the current character is 1, then we increment cnt. Otherwise, if there is a previous character and the previous character is 1, then the previous cnt number of 1s can be moved backward, and we add cnt to the answer.

Finally, we return the answer.

The time complexity is O(n), where n is the length of the string s. The space complexity is O(1).

Code

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Complexity Comparison

ApproachComplexity
Greedy Approach

Time Complexity: O(n), as we iterate through the string once.
Space Complexity: O(1), using constant extra space.

Scan and Balance Counting

Time Complexity: O(n), going through the string once.
Space Complexity: O(1), no extra space beyond constant counters.

Greedy—

Detailed Complexity Analysis

ApproachTimeSpaceWhen to Use
Greedy CountingO(n)O(1)Best general solution. Single pass counting of 1-before-0 pairs.
Scan and Balance CountingO(n)O(1)Useful when reasoning about active 1s moving across zeros during the scan.

Video Solution

Maximum Number of Operations to Move Ones to the End | Interview Style | Leetcode 3228 | MIK • codestorywithMIK • 6,513 views views

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Frequently Asked Questions

Is Maximum Number of Operations to Move Ones to the End easy or hard?
This problem is generally classified as Medium difficulty. The implementation is short, but the key insight is recognizing that the number of operations equals the count of 1-before-0 pairs in the string. Once that observation is made, the solution becomes a straightforward linear scan.
Maximum Number of Operations to Move Ones to the End Python/Java solution
The implementation in Python or Java follows the same logic: iterate through the string, maintain a running count of 1s, and add that count to the answer whenever a 0 is encountered. Both implementations run in O(n) time and require constant additional space.
How to solve Maximum Number of Operations to Move Ones to the End in O(n)?
Iterate through the string from left to right. Maintain a counter for how many 1s have appeared so far. When encountering a 0, add the number of seen 1s to the result because those 1s will eventually move past that 0. Continue scanning until the end to compute the total operations.
What is the best approach for Maximum Number of Operations to Move Ones to the End?
The greedy counting approach is the most efficient and easiest to reason about. Scan the string once while counting how many 1s have appeared so far. Each time a 0 appears, add the current number of 1s to the total operations. This directly counts all (1,0) pairs in O(n) time and O(1) space.
Is Maximum Number of Operations to Move Ones to the End asked at Google/Amazon/Meta?
Problems involving inversion counting, greedy scans, and binary string manipulation are common in interviews at companies like Google, Amazon, and Meta. This specific pattern—counting 1-before-0 pairs using a linear scan—appears frequently in medium-level coding interviews.
What data structure is used in Maximum Number of Operations to Move Ones to the End?
No complex data structures are required. The optimal solution uses simple integer counters while scanning a string. The technique falls under greedy algorithms and counting patterns rather than relying on stacks, heaps, or hash maps.
What is the time complexity of Maximum Number of Operations to Move Ones to the End?
The optimal solution runs in O(n) time where n is the length of the binary string. The algorithm performs a single pass through the string while maintaining a counter for previously seen 1s. Space complexity remains O(1) because only a few integer variables are required.

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