Added 941-valid-mountain-array.

dsa-solutions/lc-solutions/0900-0999/941-valid-mountain-array.md

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+---
+id: valid-mountain-array
+title: Valid Mountain Array
+level: easy
+sidebar_label: Valid Mountain Array
+tags:
+  - Array
+description: "This document provides a solution for the Valid Mountain Array problem."
+---
+
+## Problem Statement
+
+Given an array of integers `arr`, return `true` if and only if it is a valid mountain array.
+
+Recall that `arr` is a mountain array if and only if:
+
+- `arr.length >= 3`
+- There exists some `i` 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]`
+
+**Example 1:**
+
+Input: `arr = [2, 1]`
+
+Output: `false`
+
+**Example 2:**
+
+Input: `arr = [3, 5, 5]`
+
+Output: `false`
+
+**Example 3:**
+
+Input: `arr = [0, 3, 2, 1]`
+
+Output: `true`
+
+**Constraints:**
+
+- `1 <= arr.length <= 10^4`
+- `0 <= arr[i] <= 10^4`
+
+## Solutions
+
+### Approach
+
+To determine if the given array is a valid mountain array, follow these steps:
+
+1. **Initial Check**: Ensure the length of the array is at least 3.
+2. **Increasing Phase**: Traverse the array from the start, checking if each element is less than the next one.
+3. **Peak Check**: If the peak is at the beginning or end of the array, it's not a valid mountain.
+4. **Decreasing Phase**: Continue traversing from the peak, checking if each element is greater than the next one.
+5. **Final Check**: Ensure that the entire array was traversed.
+
+### Java 
+
+```java
+class Solution {
+    public boolean validMountainArray(int[] arr) {
+        int len = arr.length;
+        if (len < 3) {
+            return false;
+        }
+
+        boolean flag = true;
+        for (int i = 0; i < len - 1; i++) {
+            if (arr[i] == arr[i + 1]) {
+                return false;
+            }
+            if (flag && arr[i + 1] < arr[i]) {
+                if (i == 0) return false;
+                flag = false;
+            }
+            if (!flag && arr[i] < arr[i + 1]) {
+                return false;
+            }
+        }
+        if (flag) {
+            return false;
+        }
+        return true;
+    }
+}
+
+```
+
+### Python 
+```Python
+class Solution:
+    def validMountainArray(self, arr: List[int]) -> bool:
+        length = len(arr)
+        if length < 3:
+            return False
+
+        flag = True
+        for i in range(length - 1):
+            if arr[i] == arr[i + 1]:
+                return False
+            if flag and arr[i + 1] < arr[i]:
+                if i == 0:
+                    return False
+                flag = False
+            if not flag and arr[i] < arr[i + 1]:
+                return False
+
+        if flag:
+            return False
+
+        return True
+```