Valid Mountain Array - Practice Coding Problems

Valid Mountain Array - Problem

Array Easy

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]

This means the array must strictly increase to a peak, then strictly decrease.

Input & Output

Example 1 — Valid Mountain

$ Input: arr = [0,1,2,3,4,5,4,3,2,1,0]

› Output: true

💡 Note: This array strictly increases from 0 to 5, then strictly decreases from 5 to 0, forming a valid mountain with peak at index 5.

Example 2 — No Peak

$ Input: arr = [2,1]

› Output: false

💡 Note: Array length is less than 3, so cannot be a mountain. Also, it only decreases without any increase.

Example 3 — Only Increasing

$ Input: arr = [1,2,3,4,5]

› Output: false

💡 Note: Array only increases without any decreasing part. A mountain must have both uphill and downhill sections.

Constraints

1 ≤ arr.length ≤ 10⁴
0 ≤ arr[i] ≤ 10⁴

Visualization

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Understanding the Visualization

Input Array

Given array that may or may not be a mountain

Check Pattern

Must strictly increase to peak, then strictly decrease

Output

Return true if valid mountain, false otherwise

Key Takeaway

🎯 Key Insight: A mountain has exactly one peak with no plateaus or multiple peaks

Asked in

G Google 15 a Amazon 12

The key insight is that a valid mountain must have exactly one peak with strict increases before and strict decreases after. Best approach is two pointers to walk up then down in one pass. Time: O(n), Space: O(1)

Common Approaches

Approach	Time	Space	Notes
✓ Two Pointers - Walk Up Then Down	O(n)	O(1)	Start from beginning, walk uphill as far as possible, then walk downhill
Brute Force - Check All Conditions	O(n)	O(1)	Verify each condition separately by scanning the array multiple times

Two Pointers - Walk Up Then Down — Algorithm Steps

Walk up from start while strictly increasing
Walk down from peak while strictly decreasing
Check if we covered the entire array

Visualization

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Step-by-Step Walkthrough

Walk Up

Move pointer while values increase

Check Peak

Ensure peak is not at boundaries

Walk Down

Continue walking while values decrease

Code -

solution.c — C

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <stdbool.h>

bool solution(int* arr, int size) {
    if (size < 3) {
        return false;
    }
    
    int i = 0;
    
    // Walk up
    while (i < size - 1 && arr[i] < arr[i + 1]) {
        i++;
    }
    
    // Peak cannot be first or last element
    if (i == 0 || i == size - 1) {
        return false;
    }
    
    // Walk down
    while (i < size - 1 && arr[i] > arr[i + 1]) {
        i++;
    }
    
    // Should reach the end
    return i == size - 1;
}

int main() {
    char line[10000];
    fgets(line, sizeof(line), stdin);
    
    int arr[1000];
    int size = 0;
    char* token = strtok(line, "[,]\n");
    
    while (token != NULL) {
        if (strlen(token) > 0 && token[0] != '[' && token[0] != ']') {
            arr[size++] = atoi(token);
        }
        token = strtok(NULL, "[,]\n");
    }
    
    bool result = solution(arr, size);
    printf(result ? "true\n" : "false\n");
    
    return 0;
}

Time & Space Complexity

Time Complexity

⏱️

O(n)

Single pass through array with two phases

✓ Linear Growth

Space Complexity

O(1)

Only using pointer variables

✓ Linear Space

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Input & Output

Constraints

Visualization

Related Problems

Common Approaches

Two Pointers - Walk Up Then Down — Algorithm Steps

Visualization

Code -

Time & Space Complexity

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