vanyalanterns

codeforces/problems/vanyalanterns/vanyalanterns.cpp

@@ -0,0 +1,60 @@
+// https://codeforces.com/contest/492/problem/B
+
+#include <bits/stdc++.h>
+using namespace std;
+
+#define MAX_N 1000
+#define ll long long
+
+int n, l;
+vector<ll> lanterns(MAX_N);
+
+bool works(ll d)
+{
+  // we know that 0..lanterns[0] is lit and that lanterns[n - 1]..l is lit
+  // because it is the minimum in the binsearch
+  ll litSoFar = lanterns[0] + d;
+  for (int i = 1; i < n; ++i)
+  {
+    // light from this lantern will not reach to cover everything
+    // from what has already been covered to this lantern
+    if (d < lanterns[i] && lanterns[i] - d > litSoFar)
+      return false;
+
+    litSoFar = lanterns[i] + d;
+  }
+
+  return true;
+}
+
+int main()
+{
+  cin >> n >> l;
+
+  l *= 2;
+
+  for (int i = 0; i < n; ++i)
+    cin >> lanterns[i], lanterns[i] *= 2;
+
+  sort(lanterns.begin(), lanterns.begin() + n);
+
+  ll lo = max(lanterns[0], l - lanterns[n - 1]),
+     hi = l + 5, mid, ans = -1;
+  while (lo <= hi)
+  {
+    mid = lo + (hi - lo) / 2;
+    if (works(mid))
+    {
+      ans = mid;
+      hi = mid - 1;
+    }
+    else
+      lo = mid + 1;
+  }
+
+  assert(ans != -1);
+
+  cout << ans / 2 << '.' << (ans % 2 == 0 ? '0' : '5') << "000000000\n";
+
+  return 0;
+}