二分算法

while(l + 1 < r) {

　　int mid = l + r >> 1;

　　if(check(mid)) l = mid;

　　else r = mid;

}

class Solution {
public:
    int findRadius(vector<int>& houses, vector<int>& heaters) {
        int n = houses.size(), m = heaters.size();

        sort(houses.begin(), houses.end());
        sort(heaters.begin(), heaters.end());

        auto check = [&](int rid) -> bool {
            for(int i = 0, j = 0; i < n; i ++ ) {
                while(j < m && houses[i] > heaters[j] + rid) j ++ ;
                if(j < m && heaters[j] - rid <= houses[i] && heaters[j] + rid >= houses[i]) continue;
                return false;
            }
            return true;
        };

        int l = -1, r = 1e9;
        while(l + 1 < r) {
            int mid = l + r >> 1;
            if(check(mid)) r = mid;
            else l = mid;
        }

        return r;
    }
};