Question

845. Longest Mountain in Array

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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

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

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;
  }
}

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;
  }
};

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 := 1; i < n; i++ {
    if arr[i] > arr[i-1] {
      f[i] = f[i-1] + 1
    }
  }
  for i := n - 2; i >= 0; 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
}

Solution 2

class Solution:
  def longestMountain(self, arr: List[int]) -> int:
    n = len(arr)
    ans = l = 0
    while l + 2 < n:
      r = l + 1
      if arr[l] < arr[r]:
        while r + 1 < n and arr[r] < arr[r + 1]:
          r += 1
        if r < n - 1 and arr[r] > arr[r + 1]:
          while r < n - 1 and arr[r] > arr[r + 1]:
            r += 1
          ans = max(ans, r - l + 1)
        else:
          r += 1
      l = r
    return ans

class Solution {
  public int longestMountain(int[] arr) {
    int n = arr.length;
    int ans = 0;
    for (int l = 0, r = 0; l + 2 < n; l = r) {
      r = l + 1;
      if (arr[l] < arr[r]) {
        while (r + 1 < n && arr[r] < arr[r + 1]) {
          ++r;
        }
        if (r + 1 < n && arr[r] > arr[r + 1]) {
          while (r + 1 < n && arr[r] > arr[r + 1]) {
            ++r;
          }
          ans = Math.max(ans, r - l + 1);
        } else {
          ++r;
        }
      }
    }
    return ans;
  }
}

class Solution {
public:
  int longestMountain(vector<int>& arr) {
    int n = arr.size();
    int ans = 0;
    for (int l = 0, r = 0; l + 2 < n; l = r) {
      r = l + 1;
      if (arr[l] < arr[r]) {
        while (r + 1 < n && arr[r] < arr[r + 1]) {
          ++r;
        }
        if (r + 1 < n && arr[r] > arr[r + 1]) {
          while (r + 1 < n && arr[r] > arr[r + 1]) {
            ++r;
          }
          ans = max(ans, r - l + 1);
        } else {
          ++r;
        }
      }
    }
    return ans;
  }
};

func longestMountain(arr []int) (ans int) {
  n := len(arr)
  for l, r := 0, 0; l+2 < n; l = r {
    r = l + 1
    if arr[l] < arr[r] {
      for r+1 < n && arr[r] < arr[r+1] {
        r++
      }
      if r+1 < n && arr[r] > arr[r+1] {
        for r+1 < n && arr[r] > arr[r+1] {
          r++
        }
        ans = max(ans, r-l+1)
      } else {
        r++
      }
    }
  }
  return
}