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1762 - Buildings With an Ocean View (Medium)

https://leetcode.com/problems/buildings-with-an-ocean-view/

Problem Statement​

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.

Constraints:

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

Approach 1: Backwards​

We iterate heightsheights from right to left and track the maximum height. If the current building is taller than the maximum height, it means it has an ocean view and we can set it as the new max height.

Since we do it backwards, we reverse the answer at the end.

Written by @wingkwong
class Solution {
public:
    vector<int> findBuildings(vector<int>& heights) {
        int n = heights.size(), prev_mx = -1;
        vector<int> ans;
        // do it backwards
        for (int i = n - 1; i >= 0; i--) {
            // check if the current building is taller than the prev max
            if (heights[i] > prev_mx) {
                // if so, then this buildign has an ocean view
                ans.push_back(i);
                // update prev_mx
                prev_mx = heights[i];
            }
        }
        // reverse the order
        reverse(ans.begin(), ans.end());
        return ans;
    }
};