Buildings with Sunlight

Problem Summary

You are given an array arr[] where each element represents the height of a building.

Sunlight comes from the left side.

A building can see sunlight if:

There is no taller building before it (to its left)

Important:

Buildings with equal height DO NOT block each other

Key Insight

A building gets sunlight if its height is:

greater than or equal to the maximum height seen so far from the left

Concept (Greedy Approach)

We traverse the array from left to right and track:

maxHeight → tallest building seen so far
count → number of buildings receiving sunlight

Rule:

If:

current height >= maxHeight

It gets sunlight.

Then:

Increase count
Update maxHeight

Example Walkthrough

Input

arr = [6, 2, 8, 4, 11, 13]

Step-by-step

Index	Height	maxHeight	Gets Sunlight?	Count
0	6	6	Yes	1
1	2	6	No	1
2	8	8	Yes	2
3	4	8	No	2
4	11	11	Yes	3
5	13	13	Yes	4

Output

Another Example

Input

arr = [3, 3, 3, 1]

Height	Condition	Result
3	First building	YES
3	Equal height	YES
3	Equal height	YES
1	Smaller	NO

Output

Algorithm (Simple Steps)

Initialize:
- maxHeight = 0
- count = 0
Traverse array:
- If arr[i] >= maxHeight:
  - count++
  - maxHeight = arr[i]
Return count

Java Code

class Solution {
    public int visibleBuildings(int arr[]) {
        int maxHeight = 0;
        int count = 0;

        for (int height : arr) {
            if (height >= maxHeight) {
                count++;
                maxHeight = height;
            }
        }

        return count;
    }
}

Complexity

Time Complexity: O(n)
→ Single pass through array
Space Complexity: O(1)
→ No extra space used

Key Takeaways

This is a greedy + traversal problem
Track the maximum so far
Equal heights are allowed (important edge case!)
Very common in interviews (Amazon, Microsoft)