Submission #313455

#	Time	Username	Problem	Language	Result	Execution time	Memory
313455		VROOM_VARUN	Railway Trip (JOI17_railway_trip)	C++14	100 / 100	364 ms	23684 KiB

railway_trip

/*
ID: varunra2
LANG: C++
TASK: railways
*/

#include<bits/stdc++.h>
using namespace std;

#ifdef DEBUG
#include "lib/debug.h"
#define debug(...) cerr << "[" << #__VA_ARGS__ << "]:", debug_out(__VA_ARGS__)
#define debug_arr(...) \
  cerr << "[" << #__VA_ARGS__ << "]:", debug_arr(__VA_ARGS__)
#pragma GCC diagnostic ignored "-Wsign-compare"
//#pragma GCC diagnostic ignored "-Wunused-parameter"
//#pragma GCC diagnostic ignored "-Wunused-variable"
#else
#define debug(...) 42
#endif

#define EPS 1e-9
#define IN(A, B, C) assert(B <= A && A <= C)
#define INF (int)1e9
#define MEM(a, b) memset(a, (b), sizeof(a))
#define MOD 1000000007
#define MP make_pair
#define PB push_back
#define all(cont) cont.begin(), cont.end()
#define rall(cont) cont.end(), cont.begin()
#define x first
#define y second

const double PI = acos(-1.0);
typedef long long ll;
typedef long double ld;
typedef pair<int, int> PII;
typedef map<int, int> MPII;
typedef multiset<int> MSETI;
typedef set<int> SETI;
typedef set<string> SETS;
typedef vector<int> VI;
typedef vector<PII> VII;
typedef vector<VI> VVI;
typedef vector<string> VS;

#define rep(i, a, b) for(int i = a; i < (b); ++i)
#define trav(a, x) for(auto& a : x)
#define sz(x) (int)(x).size()
typedef pair<int, int> pii;
typedef vector<int> vi;
#pragma GCC diagnostic ignored "-Wsign-compare"
// util functions

const int siz = 20;
int n, k, q;

vector<VII> st;
VI vals;
VI prv;
VI nxt;

void init() {
  st.resize(n);
  vals.resize(n);
  prv.resize(n);
  nxt.resize(n);
  for(int i = 0; i < n; i++) {
    st[i].resize(siz);
  }
}

inline PII comb(PII a, int i) {
  // combining two ranges -> min of lefts/max of rights
  return MP(min(st[a.x][i].x, st[a.y][i].x), max(st[a.x][i].y, st[a.y][i].y));
}

pair<int, PII> qry(int ind, int l, int r) {
  // what is the longest range originating form ind, that is within range l -> r?
  // also what is the minimum # of steps we need to take to reach it

  pair<int, PII> ret = MP(0, MP(ind, ind));

  for(int i = siz - 1; i >= 0; i--) {
    PII cur = comb(ret.y, i);
    if(cur.x > l and cur.y < r) {
      ret.x += (1 << i);
      ret.y = cur;
    }
  }

  return ret;
}

int getDist(int a, int b) {
  if(a > b) swap(a, b); // answer will still stay the same, because graph is undirected

  auto ran1 = qry(a, -1, b); // without going into b's "territory", longest range
  auto ran2 = qry(b, ran1.y.y, n); // now in my territory

  return ran1.x + ran2.x + 1; // + 1 because you need to merge both ranges i think

}

void calcPrv() {
  // calculate previous greater or equal element index
  stack<int> s;

  for(int i = 0; i < n; i++) {
    while(!s.empty() and vals[s.top()] < vals[i]) s.pop();
    if(s.empty()) prv[i] = -1;
    else prv[i] = s.top();
    s.push(i);
  }

  prv[0] = 0;
}

void calcNxt() {
  // calculates next greater or equal element index

  stack<int> s;

  for(int i = n - 1; i >= 0; i--) {
    while(!s.empty() and vals[s.top()] < vals[i]) s.pop();
    if(s.empty()) nxt[i] = n;
    else nxt[i] = s.top();
    s.push(i);
  }

  nxt[n - 1] = n - 1;
}

int main() {
// #ifndef ONLINE_JUDGE
  // freopen("railways.in", "r", stdin);
  // freopen("railways.out", "w", stdout);
// #endif
  cin.sync_with_stdio(0); cin.tie(0);

  cin >> n >> k >> q;

  init();

  for(int i = 0; i < n; i++) {
    cin >> vals[i];
  }

  calcPrv();
  calcNxt();

  // for simple O(n ^ 2) solution, you can make an undirected unweighted graph out of prv/nxt, and answer is shortest path from a -> b

  for(int i = 0; i < n; i++) {
    st[i][0] = MP(prv[i], nxt[i]);
  }

  for(int j = 1; j < siz; j++) {
    for(int i = 0; i < n; i++) {
      st[i][j] = comb(st[i][j - 1], j - 1);
    }
  }

  while(q--) {
    int a, b;
    cin >> a >> b;
    a--, b--;

    cout << getDist(a, b) - 1 << '\n'; // minus one because ending node doesn't count
  }

  return 0;
}

• Subtask 1: 5 / 5
• Subtask 2: 15 / 15
• Subtask 3: 25 / 25
• Subtask 4: 55 / 55