2237 - Count Positions on Street With Required Brightness (Medium)

Problem Link

https://leetcode.com/problems/count-positions-on-street-with-required-brightness/

Problem Statement

You are given an integer n. A perfectly straight street is represented by a number line ranging from 0 to n - 1. You are given a 2D integer array lights representing the street lamp(s) on the street. Each lights[i] = [positioni, rangei] indicates that there is a street lamp at position positioni that lights up the area from [max(0, positioni - rangei), min(n - 1, positioni + rangei)] (inclusive).

The brightness of a position p is defined as the number of street lamps that light up the position p. You are given a 0-indexed integer array requirement of size n where requirement[i] is the minimum brightness of the ith position on the street.

Return the number of positions i on the street between 0 and n - 1 that have a brightness of at least _ requirement[i]._

Example 1:

Input: n = 5, lights = [[0,1],[2,1],[3,2]], requirement = [0,2,1,4,1]
Output: 4
Explanation:
- The first street lamp lights up the area from [max(0, 0 - 1), min(n - 1, 0 + 1)] = [0, 1] (inclusive).
- The second street lamp lights up the area from [max(0, 2 - 1), min(n - 1, 2 + 1)] = [1, 3] (inclusive).
- The third street lamp lights up the area from [max(0, 3 - 2), min(n - 1, 3 + 2)] = [1, 4] (inclusive).

- Position 0 is covered by the first street lamp. It is covered by 1 street lamp which is greater than requirement[0].
- Position 1 is covered by the first, second, and third street lamps. It is covered by 3 street lamps which is greater than requirement[1].
- Position 2 is covered by the second and third street lamps. It is covered by 2 street lamps which is greater than requirement[2].
- Position 3 is covered by the second and third street lamps. It is covered by 2 street lamps which is less than requirement[3].
- Position 4 is covered by the third street lamp. It is covered by 1 street lamp which is equal to requirement[4].

Positions 0, 1, 2, and 4 meet the requirement so we return 4.

Example 2:

Input: n = 1, lights = [[0,1]], requirement = [2]
Output: 0
Explanation:
- The first street lamp lights up the area from [max(0, 0 - 1), min(n - 1, 0 + 1)] = [0, 0] (inclusive).
- Position 0 is covered by the first street lamp. It is covered by 1 street lamp which is less than requirement[0].
- We return 0 because no position meets their brightness requirement.

Constraints:

• 1 <= n <= 10^5
• 1 <= lights.length <= 10^5
• 0 <= positioni < n
• 0 <= rangei <= 10^5
• requirement.length == n
• 0 <= requirement[i] <= 10^5

Approach 1: Line Sweep

Written by @wingkwong

class Solution {
public:
    int meetRequirement(int n, vector<vector<int>>& lights, vector<int>& requirement) {
        vector<int> brightness(n + 1);
        for (auto x : lights) {
            // start: + 1
            brightness[max(0, x[0] - x[1])]++;
            // end: -1
            brightness[min(n, x[0] + x[1] + 1)]--;
        }
        // calculate the prefix sum
        partial_sum(brightness.begin(), brightness.end(), brightness.begin());
        int ans = 0;
        // check each point's brightness to see if it meets its requirement
        for (int i = 0; i < n; i++) ans += brightness[i] >= requirement[i];
        return ans;
    }
};