poj 3662

#include<algorithm>
#include<bitset>
#include<cctype>
#include<cerrno>
#include<clocale>
#include<cmath>
#include<complex>
#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<ctime>
#include<deque>
#include<exception>
#include<fstream>
#include<functional>
#include<limits>
#include<list>
#include<map>
#include<iomanip>
#include<ios>
#include<iosfwd>
#include<iostream>
#include<istream>
#include<ostream>
#include<queue>
#include<set>
#include<sstream>
#include<stack>
#include<stdexcept>
#include<streambuf>
#include<string>
#include<utility>
#include<vector>
#include<cwchar>
#include<cwctype>
using namespace std;
const int maxn = 100009;
const int maxxn = 1009;
#define inf 0x3f3f3f
queue<int > q;

int tot = 0;
int n,m,k;

int maxx = -100;
int minn = 10000000;
int mid;
int l,r;
int head[maxxn];
int d[maxxn];
int v[maxxn];
struct node {
	int fr,ver,value,Next;
};
node edge[maxn];

inline bool spfa () {
	memset(d,inf,sizeof(d));
	memset(v,0,sizeof(v));
	d[1] = 0;
	q.push(1);
	v[1] = 1;
	while (!q.empty()) {
		int x = q.front();
		q.pop();
		v[x] = 0;
		for (int i = head[x];i;i = edge[i].Next) {
			int y = edge[i].ver;
			int z = edge[i].value;
			if (z > mid) z = 1;
			else z = 0;
			if (d[y] > d[x] + z) {
				d[y] = d[x] + z;
				if (v[y] == 0) {
					q.push(y);
					v[y] = 1;
				} 
			}
		}
	}
	if (d[n] <= k) return 1;
	else return 0;
}
inline void add (int x,int y,int z) {
	edge[++tot].ver = y; edge[tot].fr = x; edge[tot].value = z; edge[tot].Next = head[x]; head[x] = tot;
}
inline int Read () {
	int xx = 0;
	int ff = 1;
	char ch = getchar();
	while (ch < '0'||ch > '9') {
		if (ch == '-') ff = -1;
		ch = getchar();
	}
	while (ch >= '0'&&ch <= '9') {
		xx = (xx << 1) + (xx << 3) + ch - '0';
		ch = getchar();
	}
	return xx * ff;
} 

int main () {
	n = Read(); m = Read(); k = Read();
	for (int i = 1;i <= m;i++) {
		int a = Read(); int b = Read(); int c = Read();
		maxx = max(c,maxx);
		add(a,b,c); add(b,a,c);
	}
	l = 0;
	r = maxx;
	int ans = -1;
	while (l <= r) {
		mid = (l + r) / 2;
		if (spfa() == 1) {
			ans = mid,r = mid - 1;
		}
		else {
			l = mid + 1;
		}
	}
	printf ("%d\n",ans);
	return 0; 
}

但是下面的这个就挂了

二分是有一定的使用技巧的

二分的最后一个mid值不一定合法，（因为即使不合法，l与r也会更新，而只要满足l>=r就会跳出）所以一定要记录最后一个合法的mid

#include<algorithm>
#include<bitset>
#include<cctype>
#include<cerrno>
#include<clocale>
#include<cmath>
#include<complex>
#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<ctime>
#include<deque>
#include<exception>
#include<fstream>
#include<functional>
#include<limits>
#include<list>
#include<map>
#include<iomanip>
#include<ios>
#include<iosfwd>
#include<iostream>
#include<istream>
#include<ostream>
#include<queue>
#include<set>
#include<sstream>
#include<stack>
#include<stdexcept>
#include<streambuf>
#include<string>
#include<utility>
#include<vector>
#include<cwchar>
#include<cwctype>
using namespace std;
const int maxn = 100009;
const int maxxn = 1009;
#define inf 0x3f3f3f
queue<int > q;

int tot = 0;
int n,m,k;

int maxx = -100;
int minn = 10000000;
int mid;
int l,r;
int head[maxxn];
int d[maxxn];
int v[maxxn];
struct node {
	int fr,ver,value,Next;
};
node edge[maxn];

inline bool spfa () {
	memset(d,inf,sizeof(d));
	memset(v,0,sizeof(v));
	d[1] = 0;
	q.push(1);
	v[1] = 1;
	while (!q.empty()) {
		int x = q.front();
		q.pop();
		v[x] = 0;
		for (int i = head[x];i;i = edge[i].Next) {
			int y = edge[i].ver;
			int z = edge[i].value;
			if (z > mid) z = 1;
			else z = 0;
			if (d[y] > d[x] + z) {
				d[y] = d[x] + z;
				if (v[y] == 0) {
					q.push(y);
					v[y] = 1;
				} 
			}
		}
	}
	if (d[n] <= k) return 1;
	else return 0;
}
inline void add (int x,int y,int z) {
	edge[++tot].ver = y; edge[tot].fr = x; edge[tot].value = z; edge[tot].Next = head[x]; head[x] = tot;
}
inline int Read () {
	int xx = 0;
	int ff = 1;
	char ch = getchar();
	while (ch < '0'||ch > '9') {
		if (ch == '-') ff = -1;
		ch = getchar();
	}
	while (ch >= '0'&&ch <= '9') {
		xx = (xx << 1) + (xx << 3) + ch - '0';
		ch = getchar();
	}
	return xx * ff;
} 

int main () {
	n = Read(); m = Read(); k = Read();
	for (int i = 1;i <= m;i++) {
		int a = Read(); int b = Read(); int c = Read();
		maxx = max(c,maxx);
		add(a,b,c); add(b,a,c);
	}
	l = 0;
	r = maxx;
	while (l <= r) {

		mid = (l + r) / 2;
		if (spfa() == 1) {
			r = mid - 1;
		}
		else {
			l = mid + 1;
		}
	}
	for (int i = 1;i <= tot;i++) {
		if (edge[i].value >= mid && edge[i].value <= minn) minn = edge[i].value;
	}
	printf ("%d\n",minn==10000000?-1:minn);
	return 0; 
}