Find the Peaks - Practice Coding Problems

Find the Peaks - Problem

Array Enumeration Easy

You are given a 0-indexed array mountain. Your task is to find all the peaks in the mountain array.

Return an array that consists of indices of peaks in the given array in any order.

Notes:

A peak is defined as an element that is strictly greater than its neighboring elements.
The first and last elements of the array are not a peak.

Input & Output

Example 1 — Basic Case

$ Input: mountain = [2,4,6,3,1]

› Output: [2]

💡 Note: Element at index 2 (value 6) is greater than both neighbors: 6 > 4 and 6 > 3

Example 2 — Multiple Peaks

$ Input: mountain = [1,4,3,8,5]

› Output: [1,3]

💡 Note: Index 1: 4 > 1 and 4 > 3. Index 3: 8 > 3 and 8 > 5

Example 3 — No Peaks

$ Input: mountain = [1,2,3]

› Output: []

💡 Note: Only one interior element at index 1: 2 > 1 but 2 < 3, so not a peak

Constraints

3 ≤ mountain.length ≤ 1000
1 ≤ mountain[i] ≤ 10⁴

Visualization

Tap to expand

Understanding the Visualization

Input Array

Array [2,4,6,3,1] with indices 0-4

Peak Check

Check each interior element against neighbors

Result

Return indices of all peaks found

Key Takeaway

🎯 Key Insight: A peak must be strictly greater than both left and right neighbors, and boundary elements cannot be peaks

Asked in

G Google 15 a Amazon 12 M Microsoft 8

The key insight is that only interior elements can be peaks - first and last elements are excluded by definition. Best approach is single pass comparison checking each interior element against its neighbors. Time: O(n), Space: O(1).

Common Approaches

Approach	Time	Space	Notes
✓ Single Pass Optimization	O(n)	O(1)	Same algorithm but with early termination optimizations
Brute Force - Check Each Element	O(n)	O(1)	Iterate through array and check if each element is greater than both neighbors

Single Pass Optimization — Algorithm Steps

Handle edge cases (arrays with length < 3)
Single pass from index 1 to n-2
Compare each element with both neighbors

Visualization

Tap to expand

Step-by-Step Walkthrough

Check Length

If array length < 3, return empty array

Scan Interior

Check elements from index 1 to n-2

Peak Test

Element must be > both left and right neighbors

Code -

solution.c — C

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

int* solution(int* mountain, int mountainSize, int* returnSize) {
    if (mountainSize < 3) {
        *returnSize = 0;
        return NULL; // No peaks possible
    }
    
    int* peaks = (int*)malloc(mountainSize * sizeof(int));
    int count = 0;
    
    for (int i = 1; i < mountainSize - 1; i++) {
        if (mountain[i] > mountain[i - 1] && mountain[i] > mountain[i + 1]) {
            peaks[count++] = i;
        }
    }
    
    *returnSize = count;
    return peaks;
}

int main() {
    char line[10000];
    fgets(line, sizeof(line), stdin);
    
    // Parse array
    int mountain[1000];
    int size = 0;
    char* token = strtok(line, "[],\n");
    while (token != NULL) {
        if (strlen(token) > 0 && token[0] != ' ') {
            mountain[size++] = atoi(token);
        }
        token = strtok(NULL, "[],\n");
    }
    
    int returnSize;
    int* result = solution(mountain, size, &returnSize);
    
    printf("[");
    for (int i = 0; i < returnSize; i++) {
        if (i > 0) printf(",");
        printf("%d", result[i]);
    }
    printf("]\n");
    
    if (result) free(result);
    return 0;
}

Time & Space Complexity

Time Complexity

⏱️

O(n)

Must check each interior element once, cannot be optimized further

✓ Linear Growth

Space Complexity

O(1)

Only using constant extra space for variables

✓ Linear Space

25.0K Views

Medium Frequency

~15 min Avg. Time

890 Likes

Ln 1, Col 1

Smart Actions

💡 Explanation

AI Ready

💡 Suggestion Tab to accept Esc to dismiss

// Output will appear here after running code

Code Editor Closed

Click the red button to reopen

Input & Output

Constraints

Visualization

Related Problems

Common Approaches

Single Pass Optimization — Algorithm Steps

Visualization

Code -

Time & Space Complexity

Select Compiler