LeetCode 3818: Minimum Prefix Removal to Make Array Strictly Increasing

Meta Data

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Difficulty: medium First Attempt: 2026-01-25

• Submissions: Accepted (first try)

Approach

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What remains is a suffix that must be strictly increasing. A longer suffix means a shorter removed prefix, so walk left from the end while nums[i - 1] < nums[i] (Array Template 1). Where it stops, i, is the length to remove.

My Solution

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class Solution:
    def minimumPrefixLength(self, nums: List[int]) -> int:
        i = len(nums) - 1
        while i > 0 and nums[i - 1] < nums[i]:
            i -= 1
        return i

I originally wrote while ... else: return right; with no break the else always runs, so it is equivalent, but returning directly is clearer.

Complexity

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Time O(n), space O(1).

Mistakes to Avoid

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• Strictly increasing: stop at equal values too.

Interview Explanation

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The remainder must be a strictly increasing suffix, so I walk left until the first non-increasing step; that index is the prefix length to remove. Time O(n).