Formatted question description: https://leetcode.ca/all/1762.html

1762. Buildings With an Ocean View

Level

Medium

Description

There are n buildings in a line. You are given an integer array heights of size n that represents the heights of the buildings in the line.

The ocean is to the right of the buildings. A building has an ocean view if the building can see the ocean without obstructions. Formally, a building has an ocean view if all the buildings to its right have a smaller height.

Return a list of indices (0-indexed) of buildings that have an ocean view, sorted in increasing order.

Example 1:

Input: heights = [4,2,3,1]

Output: [0,2,3]

Explanation: Building 1 (0-indexed) does not have an ocean view because building 2 is taller.

Example 2:

Input: heights = [4,3,2,1]

Output: [0,1,2,3]

Explanation: All the buildings have an ocean view.

Example 3:

Input: heights = [1,3,2,4]

Output: [3]

Explanation: Only building 3 has an ocean view.

Example 4:

Input: heights = [2,2,2,2]

Output: [3]

Explanation: Buildings cannot see the ocean if there are buildings of the same height to its right.

Constraints:

  • 1 <= heights.length <= 10^5
  • 1 <= heights[i] <= 10^9

Solution

For index i where 0 <= i < n, the building at index i has an ocean view if and only if for all i < j < n, there is heights[i] > heights[j]. Obviously, the building at index n - 1 has an ocean view.

Loop over heights backwards and maintain the maximum height. For the building at index i, if heights[i] is greater than the maximum height, then the building has an ocean view. Update the maximum height using heights[i] after determining whether the building at index i has an ocean view.

Finally, return the indices of buildings that have an ocean view.

  • class Solution {
        public int[] findBuildings(int[] heights) {
            int length = heights.length;
            boolean[] oceanView = new boolean[length];
            int count = 0;
            int maxHeight = 0;
            for (int i = length - 1; i >= 0; i--) {
                if (heights[i] > maxHeight) {
                    oceanView[i] = true;
                    count++;
                }
                maxHeight = Math.max(maxHeight, heights[i]);
            }
            int[] buildings = new int[count];
            int index = 0;
            for (int i = 0; i < length; i++) {
                if (oceanView[i])
                    buildings[index++] = i;
            }
            return buildings;
        }
    }
    
  • // OJ: https://leetcode.com/problems/buildings-with-an-ocean-view/
    // Time: O(N)
    // Space: O(1)
    class Solution {
    public:
        vector<int> findBuildings(vector<int>& A) {
            vector<int> ans{(int)A.size() - 1};
            for (int i = A.size() - 2; i >= 0; --i) {
                if (A[i] > A[ans.back()]) ans.push_back(i);
            }
            reverse(begin(ans), end(ans));
            return ans;
        }
    };
    
  • print("Todo!")
    

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