228 lines
6.5 KiB
Text
228 lines
6.5 KiB
Text
---
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id: why-cpp
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title: Why C++?
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author: Benjamin Qi
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description: Some reasons why choice of language matters significantly outside of USACO Bronze.
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---
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## Time Limit
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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.
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- Rewriting the C++ solution for [USACO Silver Wormsort](http://www.usaco.org/index.php?page=viewproblem2&cpid=992) 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.
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<spoiler title="Python3 8/10 Solution">
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```py
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# 8/10 test cases ...
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fin = open("wormsort.in","r")
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lines = [line for line in fin]
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N,M = map(int,lines[0].split())
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p = list(map(lambda x: int(x)-1,lines[1].split()))
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ed = []
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for i in range(2,len(lines)):
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a,b,w = map(int,lines[i].split())
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a -= 1
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b -= 1
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ed.append([w,a,b])
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ed.sort()
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ed.reverse()
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adj = [[] for i in range(N)]
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vis = [0 for i in range(N)]
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cnt = 0
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def dfs(x):
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global cnt
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if vis[x] != 0:
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return
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vis[x] = cnt
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for i in adj[x]:
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dfs(i)
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def ok(mid):
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global cnt
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for i in range(N):
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vis[i] = 0
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adj[i].clear()
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for i in range(mid):
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a,b = ed[i][1],ed[i][2]
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adj[a].append(b)
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adj[b].append(a)
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for i in range(N):
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if vis[i] == 0:
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cnt += 1
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todo = [i]
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ind = 0
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while ind < len(todo):
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x = todo[ind]
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ind += 1
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vis[x] = cnt
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for i in adj[x]:
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if vis[i] == 0:
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vis[i] = -cnt
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todo.append(i)
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ok = True
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for i in range(N):
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if vis[i] != vis[p[i]]:
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ok = False
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return ok
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lo,hi = 0,M
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while lo < hi:
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mid = (lo+hi)//2
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if ok(mid):
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hi = mid
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else:
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lo = mid+1
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fout = open("wormsort.out","w")
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fout.write(str(-1 if lo == 0 else ed[lo-1][0]))
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fout.write('\n')
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```
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</spoiler>
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- A similar solution in Java requires almost 3s, which is fairly close to the time limit of 4s.
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<spoiler title="Java Solution">
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```java
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import java.io.*; // from Nick Wu
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import java.util.*;
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public class wormsort {
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public static void main(String[] args) throws IOException{
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BufferedReader br = new BufferedReader(new FileReader("wormsort.in"));
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StringTokenizer st = new StringTokenizer(br.readLine());
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int n = Integer.parseInt(st.nextToken());
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int m = Integer.parseInt(st.nextToken());
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loc = new int[n];
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component = new int[n];
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edges = new LinkedList[n];
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for(int i = 0; i < n; i++) edges[i] = new LinkedList<>();
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lhs = new int[m];
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rhs = new int[m];
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weight = new int[m];
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st = new StringTokenizer(br.readLine());
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for(int i = 0; i < n; i++) loc[i] = Integer.parseInt(st.nextToken())-1;
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for(int i = 0; i < m; i++) {
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st = new StringTokenizer(br.readLine());
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lhs[i] = Integer.parseInt(st.nextToken())-1;
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rhs[i] = Integer.parseInt(st.nextToken())-1;
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weight[i] = Integer.parseInt(st.nextToken());
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}
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br.close();
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int minW = 0;
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int maxW = 1000000001;
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while(minW != maxW) {
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int mid = (minW + maxW + 1) / 2;
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if(valid(mid)) minW = mid;
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else maxW = mid-1;
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}
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if(minW > 1e9) minW = -1;
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PrintWriter pw = new PrintWriter(new BufferedWriter(new FileWriter("wormsort.out")));
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pw.println(minW);
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pw.close();
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}
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static int[] loc, lhs, rhs, weight;
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static LinkedList<Integer>[] edges;
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static int[] component;
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private static void dfs(int curr, int label) {
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if(component[curr] == label) return;
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component[curr] = label;
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for(int child: edges[curr]) dfs(child, label);
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}
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private static boolean valid(int minW) {
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Arrays.fill(component, -1);
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for(int i = 0; i < edges.length; i++) edges[i].clear();
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for(int i = 0; i < lhs.length; i++) {
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if(weight[i] >= minW) {
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edges[lhs[i]].add(rhs[i]);
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edges[rhs[i]].add(lhs[i]);
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}
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}
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int numcomps = 0;
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for(int i = 0; i < component.length; i++) {
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if(component[i] < 0) {
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dfs(i, numcomps++);
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}
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}
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for(int i = 0; i < loc.length; i++) {
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if(component[i] != component[loc[i]]) return false;
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}
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return true;
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}
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}
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```
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</spoiler>
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- A comparable C++ solution runs in less than 700ms.
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<spoiler title="C++ Solution">
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```cpp
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#include <bits/stdc++.h>
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using namespace std;
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typedef vector<int> vi;
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const int MX = 1e5+5;
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int loc[MX], comp[MX], lhs[MX], rhs[MX], wei[MX];
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vi adj[MX];
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int n,m;
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void dfs(int cur, int label) {
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if (comp[cur] == label) return;
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comp[cur] = label;
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for (int c: adj[cur]) dfs(c,label);
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}
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bool valid(int minW) {
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for (int i = 0; i < n; ++i) {
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comp[i] = -1;
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adj[i].clear();
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}
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for (int i = 0; i < m; ++i) if (wei[i] >= minW)
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adj[lhs[i]].push_back(rhs[i]), adj[rhs[i]].push_back(lhs[i]);
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int numComps = 0;
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for (int i = 0; i < n; ++i) if (comp[i] < 0)
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dfs(i,numComps++);
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for (int i = 0; i < n; ++i)
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if (comp[i] != comp[loc[i]]) return 0;
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return 1;
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}
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int main() {
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ios_base::sync_with_stdio(0); cin.tie(0);
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freopen("wormsort.in","r",stdin);
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freopen("wormsort.out","w",stdout);
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cin >> n >> m;
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for (int i = 0; i < n; ++i) cin >> loc[i], loc[i] --;
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for (int i = 0; i < m; ++i) {
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cin >> lhs[i], lhs[i] --;
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cin >> rhs[i], rhs[i] --;
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cin >> wei[i];
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}
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int minW = 0, maxW = (int)1e9+1;
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while (minW != maxW) {
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int mid = (minW+maxW+1)/2;
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if (valid(mid)) minW = mid;
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else maxW = mid-1;
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}
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if (minW > 1e9) minW = -1;
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cout << minW;
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}
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```
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</spoiler>
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## Other Notes
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- USACO problemsetters don't always test Java (and rarely Python) solutions when setting constraints.
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- 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.
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- Java lacks features such as `#define`, `typedef`, and `auto` that are present in C++ (which some contestants rely on extensively, see "macros").
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