How do you approach Max Consecutive Ones?

Track the length of the current run of 1s as you scan.

What is the time complexity of Max Consecutive Ones?

Time complexity: O(n) - one pass. Space complexity: O(1) - two counters.

What are common mistakes on Max Consecutive Ones?

Shrinking with if instead of while - one removal is often not enough to restore the invariant. Recomputing the window's contents each step, which puts back the O(n²) the pattern exists to remove. Never recording the answer for a window that ends at the last index.

gitGood.dev

← Back to problems

Max Consecutive Ones

EasyAlgorithms

Description

Given a binary array `nums` (containing only `0`s and `1`s), return the maximum number of **consecutive** `1`s in the array.

Example 1

Input: nums = [1,1,0,1,1,1]

Output: 3

Explanation: The first two digits or the last three digits are consecutive 1s. The maximum number of consecutive 1s is 3.

Example 2

Input: nums = [1,0,1,1,0,1]

Output: 2

Example 3

Input: nums = [0,0,0]

Output: 0

Explanation: There are no 1s at all.

Constraints

- 1 <= nums.length <= 10^5 - nums[i] is either 0 or 1.

Max Consecutive Ones: approach, complexity and pitfalls

An easy sliding window problem, graded against 7 test cases (4 of them hidden).

The pattern: Sliding Window

A moving window over a contiguous run, expanding and contracting to hold an invariant.

Reach for it when you see: "Longest/shortest contiguous subarray or substring satisfying X."

More Sliding Windowproblems →

If you are stuck

Show the 3-step hint ladder

Track the length of the current run of 1s as you scan.
Reset the current run to 0 whenever you see a 0.
Keep a separate variable for the best (longest) run seen so far.

The optimal approach

Show the approach: Single Pass with a Running Counter

Scan left to right keeping two counters: the length of the current streak of 1s and the best streak seen so far. A 1 extends the current streak (and may update the best); a 0 resets the current streak to zero.

Time: O(n) - one pass
Space: O(1) - two counters

The full reference solution in every supported language stays in the editor above - reveal it there once you have had a real attempt.

What the tests check

Read off this problem's own test suite, so these are the cases a submission actually has to survive.

Single element. A one-element case is included. Two-pointer and window solutions frequently return the wrong thing when the two indices start out equal.
Duplicate values. Repeated values appear in the input, so any logic that assumes distinct elements - or that skips over equal neighbours - is exercised.
Zero. Zero appears in the input, which is the value most likely to break a truthiness check used in place of an explicit comparison.

Common Sliding Window mistakes

These apply to the pattern as a whole, not just this problem.

Shrinking with `if` instead of `while` - one removal is often not enough to restore the invariant.
Recomputing the window's contents each step, which puts back the O(n²) the pattern exists to remove.
Never recording the answer for a window that ends at the last index.