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3105. Longest Strictly Increasing or Strictly Decreasing Subarray

Description

You are given an array of integers nums. Return the length of the longest subarray of nums which is either strictly increasing or strictly decreasing.

 

Example 1:

Input: nums = [1,4,3,3,2]

Output: 2

Explanation:

The strictly increasing subarrays of nums are [1], [2], [3], [3], [4], and [1,4].

The strictly decreasing subarrays of nums are [1], [2], [3], [3], [4], [3,2], and [4,3].

Hence, we return 2.

Example 2:

Input: nums = [3,3,3,3]

Output: 1

Explanation:

The strictly increasing subarrays of nums are [3], [3], [3], and [3].

The strictly decreasing subarrays of nums are [3], [3], [3], and [3].

Hence, we return 1.

Example 3:

Input: nums = [3,2,1]

Output: 3

Explanation:

The strictly increasing subarrays of nums are [3], [2], and [1].

The strictly decreasing subarrays of nums are [3], [2], [1], [3,2], [2,1], and [3,2,1].

Hence, we return 3.

 

Constraints:

  • 1 <= nums.length <= 50
  • 1 <= nums[i] <= 50

Solutions

Solution 1: Two Passes

We first perform a pass to find the length of the longest strictly increasing subarray, and update the answer. Then we perform another pass to find the length of the longest strictly decreasing subarray, and update the answer again.

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

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class Solution:
    def longestMonotonicSubarray(self, nums: List[int]) -> int:
        ans = t = 1
        for i, x in enumerate(nums[1:]):
            if nums[i] < x:
                t += 1
                ans = max(ans, t)
            else:
                t = 1
        t = 1
        for i, x in enumerate(nums[1:]):
            if nums[i] > x:
                t += 1
                ans = max(ans, t)
            else:
                t = 1
        return ans

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class Solution {
    public int longestMonotonicSubarray(int[] nums) {
        int ans = 1;
        for (int i = 1, t = 1; i < nums.length; ++i) {
            if (nums[i - 1] < nums[i]) {
                ans = Math.max(ans, ++t);
            } else {
                t = 1;
            }
        }
        for (int i = 1, t = 1; i < nums.length; ++i) {
            if (nums[i - 1] > nums[i]) {
                ans = Math.max(ans, ++t);
            } else {
                t = 1;
            }
        }
        return ans;
    }
}

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class Solution {
public:
    int longestMonotonicSubarray(vector<int>& nums) {
        int ans = 1;
        for (int i = 1, t = 1; i < nums.size(); ++i) {
            if (nums[i - 1] < nums[i]) {
                ans = max(ans, ++t);
            } else {
                t = 1;
            }
        }
        for (int i = 1, t = 1; i < nums.size(); ++i) {
            if (nums[i - 1] > nums[i]) {
                ans = max(ans, ++t);
            } else {
                t = 1;
            }
        }
        return ans;
    }
};

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func longestMonotonicSubarray(nums []int) int {
    ans := 1
    t := 1
    for i, x := range nums[1:] {
        if nums[i] < x {
            t++
            ans = max(ans, t)
        } else {
            t = 1
        }
    }
    t = 1
    for i, x := range nums[1:] {
        if nums[i] > x {
            t++
            ans = max(ans, t)
        } else {
            t = 1
        }
    }
    return ans
}

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function longestMonotonicSubarray(nums: number[]): number {
    let ans = 1;
    for (let i = 1, t = 1; i < nums.length; ++i) {
        if (nums[i - 1] < nums[i]) {
            ans = Math.max(ans, ++t);
        } else {
            t = 1;
        }
    }
    for (let i = 1, t = 1; i < nums.length; ++i) {
        if (nums[i - 1] > nums[i]) {
            ans = Math.max(ans, ++t);
        } else {
            t = 1;
        }
    }
    return ans;
}

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