674. Longest Continuous Increasing Subsequence

Description

Given an unsorted array of integers nums, return the length of the longest continuous increasing subsequence (i.e. subarray). The subsequence must be strictly increasing.

A continuous increasing subsequence is defined by two indices l and r (l < r) such that it is [nums[l], nums[l + 1], ..., nums[r - 1], nums[r]] and for each l <= i < r, nums[i] < nums[i + 1].

Example 1:

Input: nums = [1,3,5,4,7]
Output: 3
Explanation: The longest continuous increasing subsequence is [1,3,5] with length 3.
Even though [1,3,5,7] is an increasing subsequence, it is not continuous as elements 5 and 7 are separated by element
4.

Example 2:

Input: nums = [2,2,2,2,2]
Output: 1
Explanation: The longest continuous increasing subsequence is [2] with length 1. Note that it must be strictly
increasing.

Constraints:

1 <= nums.length <= 10⁴
-10⁹ <= nums[i] <= 10⁹

Solutions

Solution 1: One-pass Scan

We can traverse the array $nums$, using a variable $cnt$ to record the length of the current consecutive increasing sequence. Initially, $cnt = 1$.

Then, we start from index $i = 1$ and traverse the array $nums$ to the right. Each time we traverse, if $nums[i - 1] < nums[i]$, it means that the current element can be added to the consecutive increasing sequence, so we set $cnt = cnt + 1$, and then update the answer to $ans = \max(ans, cnt)$. Otherwise, it means that the current element cannot be added to the consecutive increasing sequence, so we set $cnt = 1$.

After the traversal ends, we return the answer $ans$.

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

Python Code

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

Java Code

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

C++ Code

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

Go Code

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

TypeScript Code

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

Rust Code

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impl Solution {
    pub fn find_length_of_lcis(nums: Vec<i32>) -> i32 {
        let mut ans = 1;
        let mut cnt = 1;
        for i in 1..nums.len() {
            if nums[i - 1] < nums[i] {
                ans = ans.max(cnt + 1);
                cnt += 1;
            } else {
                cnt = 1;
            }
        }
        ans
    }
}

PHP Code

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class Solution {
    /**
     * @param Integer[] $nums
     * @return Integer
     */
    function findLengthOfLCIS($nums) {
        $ans = 1;
        $cnt = 1;
        for ($i = 1; $i < count($nums); ++$i) {
            if ($nums[$i - 1] < $nums[$i]) {
                $ans = max($ans, ++$cnt);
            } else {
                $cnt = 1;
            }
        }
        return $ans;
    }
}

Solution 2: Two Pointers

We can also use two pointers $i$ and $j$ to find each consecutive increasing sequence, and find the length of the longest consecutive increasing sequence as the answer.