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845. Longest Mountain in Array

Description

You may recall that an array arr is a mountain array if and only if:

  • arr.length >= 3
  • There exists some index i (0-indexed) with 0 < i < arr.length - 1 such that:
    • arr[0] < arr[1] < ... < arr[i - 1] < arr[i]
    • arr[i] > arr[i + 1] > ... > arr[arr.length - 1]

Given an integer array arr, return the length of the longest subarray, which is a mountain. Return 0 if there is no mountain subarray.

 

Example 1:

Input: arr = [2,1,4,7,3,2,5]
Output: 5
Explanation: The largest mountain is [1,4,7,3,2] which has length 5.

Example 2:

Input: arr = [2,2,2]
Output: 0
Explanation: There is no mountain.

 

Constraints:

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

 

Follow up:

  • Can you solve it using only one pass?
  • Can you solve it in O(1) space?

Solutions

Solution 1

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class Solution:
    def longestMountain(self, arr: List[int]) -> int:
        n = len(arr)
        f = [1] * n
        g = [1] * n
        for i in range(1, n):
            if arr[i] > arr[i - 1]:
                f[i] = f[i - 1] + 1
        ans = 0
        for i in range(n - 2, -1, -1):
            if arr[i] > arr[i + 1]:
                g[i] = g[i + 1] + 1
                if f[i] > 1:
                    ans = max(ans, f[i] + g[i] - 1)
        return ans

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class Solution {
    public int longestMountain(int[] arr) {
        int n = arr.length;
        int[] f = new int[n];
        int[] g = new int[n];
        Arrays.fill(f, 1);
        Arrays.fill(g, 1);
        for (int i = 1; i < n; ++i) {
            if (arr[i] > arr[i - 1]) {
                f[i] = f[i - 1] + 1;
            }
        }
        int ans = 0;
        for (int i = n - 2; i >= 0; --i) {
            if (arr[i] > arr[i + 1]) {
                g[i] = g[i + 1] + 1;
                if (f[i] > 1) {
                    ans = Math.max(ans, f[i] + g[i] - 1);
                }
            }
        }
        return ans;
    }
}

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class Solution {
public:
    int longestMountain(vector<int>& arr) {
        int n = arr.size();
        int f[n];
        int g[n];
        fill(f, f + n, 1);
        fill(g, g + n, 1);
        for (int i = 1; i < n; ++i) {
            if (arr[i] > arr[i - 1]) {
                f[i] = f[i - 1] + 1;
            }
        }
        int ans = 0;
        for (int i = n - 2; ~i; --i) {
            if (arr[i] > arr[i + 1]) {
                g[i] = g[i + 1] + 1;
                if (f[i] > 1) {
                    ans = max(ans, f[i] + g[i] - 1);
                }
            }
        }
        return ans;
    }
};

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func longestMountain(arr []int) (ans int) {
    n := len(arr)
    f := make([]int, n)
    g := make([]int, n)
    for i := range f {
        f[i] = 1
        g[i] = 1
    }
    for i :=