usaco-guide/content/2_General/Why_Cpp.md

id	title	author
why-cpp	Why C++?	Benjamin Qi

A few reasons why choice of language matters significantly (outside of Bronze).

Time Limit

Although both Python and Java receive two times the C++ time limit in USACO, this is not the case for most other websites (ex. CodeForces). Even with the extended time limits, Python and Java sometimes have trouble passing.

• Rewriting the C++ solution for USACO Silver Wormsort in Python receives TLE (Time Limit Exceeded) on 2/10 cases. I'm not sure whether it is possible to pass this problem with Python.

  Python3 8/10 Solution

  # 8/10 test cases ...
  
  fin = open("wormsort.in","r")
  lines = [line for line in fin]
  N,M = map(int,lines[0].split())
  p = list(map(lambda x: int(x)-1,lines[1].split()))
  
  ed = []
  for i in range(2,len(lines)):
    a,b,w = map(int,lines[i].split())
    a -= 1
    b -= 1
    ed.append([w,a,b])
  ed.sort()
  ed.reverse()
  
  adj = [[] for i in range(N)]
  vis = [0 for i in range(N)]
  cnt = 0
  
  def dfs(x):
    global cnt
    if vis[x] != 0:
      return
    vis[x] = cnt
    for i in adj[x]:
      dfs(i)
  
  def ok(mid):
    global cnt
    for i in range(N):
      vis[i] = 0
      adj[i].clear()
    for i in range(mid):
      a,b = ed[i][1],ed[i][2]
      adj[a].append(b)
      adj[b].append(a)
    for i in range(N):
      if vis[i] == 0:
        cnt += 1
        todo = [i]
        ind = 0
        while ind < len(todo):
          x = todo[ind] 
          ind += 1
          vis[x] = cnt
          for i in adj[x]:
            if vis[i] == 0:
              vis[i] = -cnt
              todo.append(i)
    ok = True
    for i in range(N):
      if vis[i] != vis[p[i]]:
        ok = False
    return ok
  
  lo,hi = 0,M
  while lo < hi:
    mid = (lo+hi)//2
    if ok(mid):
      hi = mid
    else:
      lo = mid+1
  
  fout = open("wormsort.out","w")
  
  fout.write(str(-1 if lo == 0 else ed[lo-1][0]))
  fout.write('\n')
  

• A similar solution in Java requires almost 3s, which is fairly close to the time limit of 4s.

  Java Solution

  import java.io.*; // from Nick Wu
  import java.util.*;
  public class wormsort {
    public static void main(String[] args) throws IOException{
      BufferedReader br = new BufferedReader(new FileReader("wormsort.in"));
      StringTokenizer st = new StringTokenizer(br.readLine());
      int n = Integer.parseInt(st.nextToken());
      int m = Integer.parseInt(st.nextToken());
      loc = new int[n];
      component = new int[n];
      edges = new LinkedList[n];
      for(int i = 0; i < n; i++) edges[i] = new LinkedList<>();
      lhs = new int[m];
      rhs = new int[m];
      weight = new int[m];
      st = new StringTokenizer(br.readLine());
      for(int i = 0; i < n; i++) loc[i] = Integer.parseInt(st.nextToken())-1;
      for(int i = 0; i < m; i++) {
        st = new StringTokenizer(br.readLine());
        lhs[i] = Integer.parseInt(st.nextToken())-1;
        rhs[i] = Integer.parseInt(st.nextToken())-1;
        weight[i] = Integer.parseInt(st.nextToken());
      }
      br.close();
      int minW = 0;
      int maxW = 1000000001;
      while(minW != maxW) {
        int mid = (minW + maxW + 1) / 2;
        if(valid(mid)) minW = mid;
        else maxW = mid-1;
      }
      if(minW > 1e9) minW = -1;
      PrintWriter pw = new PrintWriter(new BufferedWriter(new FileWriter("wormsort.out")));
      pw.println(minW);
      pw.close();
    }
    static int[] loc, lhs, rhs, weight;
    static LinkedList<Integer>[] edges;
    static int[] component;
    private static void dfs(int curr, int label) {
      if(component[curr] == label) return;
      component[curr] = label;
      for(int child: edges[curr]) dfs(child, label);
    }
    private static boolean valid(int minW) {
      Arrays.fill(component, -1);
      for(int i = 0; i < edges.length; i++) edges[i].clear();
      for(int i = 0; i < lhs.length; i++) {
        if(weight[i] >= minW) {
          edges[lhs[i]].add(rhs[i]);
          edges[rhs[i]].add(lhs[i]);
        }
      }
      int numcomps = 0;
      for(int i = 0; i < component.length; i++) {
        if(component[i] < 0) {
          dfs(i, numcomps++);
        }
      }
      for(int i = 0; i < loc.length; i++) {
        if(component[i] != component[loc[i]]) return false;
      }
      return true;
    }
  }
  

• A comparable C++ solution runs in less than 700ms.

  C++ Solution

  #include <bits/stdc++.h>
  using namespace std;
  
  typedef vector<int> vi;
  
  const int MX = 1e5+5;
  
  int loc[MX], comp[MX], lhs[MX], rhs[MX], wei[MX];
  vi adj[MX];
  int n,m; 
  
  void dfs(int cur, int label) {
    if (comp[cur] == label) return;
    comp[cur] = label;
    for (int c: adj[cur]) dfs(c,label);
  }
  
  bool valid(int minW) {
    for (int i = 0; i < n; ++i) {
      comp[i] = -1;
      adj[i].clear();
    }
    for (int i = 0; i < m; ++i) if (wei[i] >= minW)
      adj[lhs[i]].push_back(rhs[i]), adj[rhs[i]].push_back(lhs[i]);
    int numComps = 0;
    for (int i = 0; i < n; ++i) if (comp[i] < 0)
      dfs(i,numComps++);
    for (int i = 0; i < n; ++i) 
      if (comp[i] != comp[loc[i]]) return 0;
    return 1;
  }
  
  int main() {
    ios_base::sync_with_stdio(0); cin.tie(0);
    freopen("wormsort.in","r",stdin);
    freopen("wormsort.out","w",stdout);
    cin >> n >> m;
    for (int i = 0; i < n; ++i) cin >> loc[i], loc[i] --;
    for (int i = 0; i < m; ++i) {
      cin >> lhs[i], lhs[i] --;
      cin >> rhs[i], rhs[i] --;
      cin >> wei[i];
    }
    int minW = 0, maxW = (int)1e9+1;
    while (minW != maxW) {
      int mid = (minW+maxW+1)/2;
      if (valid(mid)) minW = mid;
      else maxW = mid-1;
    }
    if (minW > 1e9) minW = -1;
    cout << minW;
  }
  

Other Notes

• USACO problemsetters don't always test Java (and rarely Python) solutions when setting constraints.
• Python lacks a data structure that keeps its keys in sorted order (the equivalent of set in C++), which is required for some silver problems.
• Java lacks features such as #define, typedef, and auto that are present in C++ (which some contestants rely on extensively, see "macros").