Submission #1792061

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Source Code Expand

#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <iostream>
#include <algorithm>
typedef long long ll ;
#define rep(i, a, b) for (int i = a; i <= b; ++ i) 
const int N = 2e5 + 5, M = N * 2 , inf = 1e9 + 7;
using namespace std ;

bool vis[N] ;
int n, rt, S, e, ter[M], nxt[M], lnk[N], dep[N], fa[N], ans ;
struct edge {
	int x, y ;
} ed[N] ;

void add(int x, int y) {
	ter[++ e] = y, nxt[e] = lnk[x], lnk[x] = e ;
}

void dfs(int p, int las) {
	for (int i = lnk[p] ; i; i = nxt[i]) if (ter[i] != las) {
		dep[ter[i]] = dep[p] + 1 ;
		fa[ter[i]] = p ;
		dfs(ter[i], p) ;
	}
}

void dfs2(int p, int las, int len) {
	if (ans == inf) return ;
	if (dep[p] < len) return ;
	if (dep[p] == len) {
		ans = max(ans, len * 2) ;
		return ;
	}
	if (vis[p]) {
		ans = inf ;
		return ;
	}
	for (int i = lnk[p] ; i; i = nxt[i]) if (ter[i] != las) dfs2(ter[i], p, len + 1) ;
	ans = max(ans, dep[p] * 2) ;
}

bool chk(int x, int y) {
	if (dep[x] < dep[y]) swap(x, y) ;
	if (fa[x] == y) return false ;
	if (fa[fa[x]] == y) return false ;
	if (fa[x] == fa[y]) return false ;
	return true ;
}

int main() {
	scanf("%d%d%d", &n, &S, &rt) ;
	rep(i, 1, n - 1) scanf("%d%d", &ed[i].x, &ed[i].y) ;
	int x, y ;
	rep(i, 1, n - 1) {
		scanf("%d%d", &x, &y) ;
		add(x, y), add(y, x) ;
	}
	dfs(rt, 0) ;
	rep(i, 1, n - 1) if (chk(ed[i].x, ed[i].y)) vis[ed[i].x] = vis[ed[i].y] = true ;
	rep(i, 1, n) lnk[i] = 0 ;
	rep(i, 1, e) nxt[i] = 0 ; e = 0 ;
	rep(i, 1, n - 1) add(ed[i].x, ed[i].y), add(ed[i].y, ed[i].x) ;
	dfs2(S, 0, 0) ;
	if (ans == inf) {
		printf("-1\n") ;
	} else printf("%d\n", ans) ;
	return 0 ; 
}

Submission Info

Compile Error

./Main.cpp: In function ‘int main()’:
./Main.cpp:53:31: warning: ignoring return value of ‘int scanf(const char*, ...)’, declared with attribute warn_unused_result [-Wunused-result]
  scanf("%d%d%d", &n, &S, &rt) ;
                               ^
./Main.cpp:54:53: warning: ignoring return value of ‘int scanf(const char*, ...)’, declared with attribute warn_unused_result [-Wunused-result]
  rep(i, 1, n - 1) scanf("%d%d", &ed[i].x, &ed[i].y) ;
                                                     ^
./Main.cpp:57:25: warning: ignoring return value of ‘int scanf(const char*, ...)’, declared with attribute warn_unused_result [-Wunused-result]
   scanf("%d%d", &x, &y) ;
                         ^

Judge Result