2237. Count Positions on Street With Required Brightness

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].

class Solution {
public:
    int meetRequirement(int n, vector<vector<int>>& lights, vector<int>& requirement) {
        vector<int> psum(n + 1);
        for(auto& light : lights) {
            int mid = light[0], len = light[1];
            int l = max(0,mid-len), r = min(n,mid + len + 1);
            psum[l] += 1;
            psum[r] -= 1;
        }
        int res = 0;
        for(int i = 0; i < n; i++) {
            psum[i] = (i ? psum[i-1] : 0) + psum[i];
            res += (psum[i] >= requirement[i]);
        }
        return res;
    }
};