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

4

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

3

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)