二分查找 http://ac.jobdu.com/problem.php?pid=1545

#include<iostream>
#include<stdio.h>
#include<stdlib.h>
#include<string.h>
#include<vector>
#include<queue>
#include<algorithm>
using namespace std;
 
const int MaxInt = 0x7fffffff;
const int Max = 100010;
 
struct Node
{
    int next;
    int value;
    bool flag;
    Node():next(0), value(0), flag(false) { }
    Node(int nxt, int val): next(nxt), value(val), flag(false) { }
};
vector<Node> pLink[10001];
bool visited[10001];
int cost[Max];

int maxValue;
 
void setVector(int n)
{
    for (int i = 1; i <= n; ++i)
        pLink[i].clear();
}
 
bool bfs(int bgn, int end, int limt)
{
	memset(visited, 0, sizeof(visited) / sizeof(bool));
    queue<int> Q;
    Q.push(bgn);
	visited[bgn] = true;
    while (!Q.empty())
    {
        int u = Q.front();
        Q.pop();
        for (int i = 0; i < pLink[u].size(); ++i)
        {
            int v = pLink[u][i].next;
			int val = pLink[u][i].value;
			if (visited[v] || val > limt) continue;
            if (v == end) return true;
			visited[v] = true;
            Q.push(v);
        }
    }
 
    return false;
}

int main()
{
 
    //freopen("in.txt", "r", stdin);
    int n, m, a, b, c;
    while (scanf("%d %d", &n, &m) != EOF)
    {
        setVector(n);
        int cnt = 0;
        while (m--)
        {
            scanf("%d %d %d", &a, &b, &c);
            pLink[a].push_back(Node(b, c));
            pLink[b].push_back(Node(a, c));
			cost[cnt++] = c;
        }
		if (n == 1) 
		{
			printf("0
");
			continue;
		}
		sort(cost, cost + cnt);
		int *end = unique(cost, cost + cnt);
		int ans = -1;
		int L = 0;
		int R = end - cost - 1;
		while (L <= R)
		{
			int mid = (L + R)>>1;
			if (bfs(1, n, cost[mid]))
			{
				ans = cost[mid];
				R = mid - 1;
			}
			else
			{
				L = mid + 1;
			}
		}
		printf("%d
", ans);
    }
 
    return 0;
}

/* 超时算法
void dfs(int u, int cost, int n)
{
    if (u == n)
    {
        if (cost < maxValue)
        {
            maxValue = cost;
        }
        return ;
    }
    for (int i = 0; i < pLink[u].size(); ++i)
    {
        int v = pLink[u][i].next;
        int value = pLink[u][i].value;
        if(pLink[u][i].flag == true || value >= maxValue) continue;
        pLink[u][i].flag = true;
        int tmpCost = cost;
        if (value > tmpCost) tmpCost = value;
        dfs(v, tmpCost, n);
        pLink[u][i].flag = false;
    }
}
*/