978. Longest Turbulent Subarray

Medium (Trung bình) C++ 🔗 Xem trên LeetCode

📋 Đề Bài

Given an integer array arr, return the length of a maximum size turbulent subarray of arr.

A subarray is turbulent if the comparison sign flips between each adjacent pair of elements in the subarray.

More formally, a subarray [arr[i], arr[i + 1], ..., arr[j]] of arr is said to be turbulent if and only if:

  • For i <= k < j:
    • arr[k] > arr[k + 1] when k is odd, and
    • arr[k] < arr[k + 1] when k is even.
  • Or, for i <= k < j:
    • arr[k] > arr[k + 1] when k is even, and
    • arr[k] < arr[k + 1] when k is odd.

 

Example 1:

Input: arr = [9,4,2,10,7,8,8,1,9]
Output: 5
Explanation: arr[1] > arr[2] < arr[3] > arr[4] < arr[5]

Example 2:

Input: arr = [4,8,12,16]
Output: 2

Example 3:

Input: arr = [100]
Output: 1

 

Constraints:

  • 1 <= arr.length <= 4 * 104
  • 0 <= arr[i] <= 109

🧠 Thuật Toán & Kỹ Thuật

Dynamic Programming (Quy hoạch động)
⏱️ Thời gian O(n×m)
💾 Không gian O(n×m)

💻 Lời Giải

C++ 0978-longest-turbulent-subarray.cpp 
class Solution {
public:
    int maxTurbulenceSize(vector<int>& arr) {
        const int n = arr.size();
        int dp[n][2];
        memset(dp, 0, sizeof(dp));
        int maxLen = 0;
        
        for (int i = 1; i < n; ++i) {
            if (arr[i - 1] < arr[i]) {
                dp[i][1] = dp[i - 1][0] + 1;
                maxLen = max(maxLen, dp[i][1]);
            }
            else if (arr[i - 1] > arr[i]) {
                dp[i][0] = dp[i - 1][1] + 1;
                maxLen = max(maxLen, dp[i][0]);
            }
        }
        
        return maxLen + 1;
    }
};