Submission #605315

#TimeUsernameProblemLanguageResultExecution timeMemory
605315chuangsheepBalloons (CEOI11_bal)C++11
100 / 100
352 ms3376 KiB
#include <bits/stdc++.h>
using namespace std;

/*
 * how long can the radius of the new balloon at position bx be
 * so that it touches the ballon a, which is described by
 * its x position a.first and its radius a.second
 */
double calc_r(pair<double, double> a, double bx) {
  return (a.first - bx) * (a.first - bx) / (4 * a.second);
}

int main() {
  // number of balloons
  int n;
  cin >> n;

  // the radius of each balloon after inflating
  vector<double> res(n);
  // the balloons we should check when we add a new one
  stack<pair<double, double>> to_check;

  // simulate inflating balloons
  for (int i = 0; i < n; i++) {
    /*
     * the x coordinate and the maximum radius of the
     * current balloon being inflated
     */
    double x, r;
    cin >> x >> r;

    // the maximum radius of the current balloon
    double max_r = r;
    /*
     * as long as the stack is not empty, we want to check
     * if the last balloon makes the radius of the current balloon smaller
     */
    while (!to_check.empty()) {
      pair<double, double> last = to_check.top();
      // the radius if the current balloon touches the last balloon
      double to_last_r = calc_r(last, x);
      /*
       * the maximum possible radius is the smaller one between current
       * maximum and the maximum radius so that we touches the last balloon
       * (by which we have to stop)
       */
      max_r = min(max_r, to_last_r);

      /*
       * if current maximum radius >= radius of the last balloon, we can
       * remove the last balloon since it will never be touched by any
       * new balloons other than the current one
       */
      if (max_r >= last.second) {
        to_check.pop();
        // check the next balloon saved which will possibly reduce max_r
        continue;
      }
      /*
       * otherwise, the current balloon is smaller than the last saved balloon
       * and we can stop checking
       */
      else {
        break;
      }
    }
    /*
     * save the x coordinate and radius of the current balloon, so that
     * it can be checked later when a new balloon being inflated
     */
    to_check.push({x, max_r});

    res[i] = max_r;
  }

  cout << fixed << setprecision(3);
  for (auto &r : res) {
    cout << r << "\n";
  }
}
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