ccf-201703-4-地铁修建

二分+并查集

发现了一个很奇怪的问题，二分的上界设置错了，结果是运行错误。。。只有95分。后来改了上界就100了，百思不得其解

#include<cstdio>
#include<cstring>
#include<algorithm>
#include<iostream>
#include<queue>
#include<cmath>
#include<map>
#include<stack>
#include<set>
#include<bitset>

using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef pair<int, int> pii;
#define pb(x) push_back(x)
#define cls(x, val) memset(x, val, sizeof(x))
#define fi first
#define se second
#define mp(x, y) make_pair(x, y)
#define inc(i, l, r) for(int i=l; i<=r; i++)
const int inf = 0x3f3f3f3f;
const int maxn = 2e5+10;
struct Node{
    int u, v, w;
};
int n, m;
vector<Node> G;
int fa[maxn];

int find_fa(int x){
    return x == fa[x]?x:fa[x] = find_fa(fa[x]);
}

void uni(int u, int v){
    int fau = find_fa(u);
    int fav = find_fa(v);
    if(fau!=fav){
        fa[fau] = fav;
    }
}

bool dfs(int val){
    for(int i=1; i<=n; i++) fa[i] = i;
    int u, v;
    for(int i=0; i<G.size(); i++){
        if(G[i].w>val) continue;
        u = G[i].u, v = G[i].v;
        uni(u, v);
    }
    find_fa(1), find_fa(n);
    return (fa[1] == fa[n]);
}

int main(){
    ios::sync_with_stdio(false);
    cin>>n>>m;
    int u, v, w;
    for(int i=1; i<=m; i++){
        cin>>u>>v>>w;
        G.push_back(Node{u, v, w});
    }
    //很迷的一个wa点，上界设成1e9就总是95分
    int l=1, r=1000000;
    int ans;
    while(l<=r){
        int mid = (l+r)/2;
        //cout<<l<<" "<<r<<endl;
        if(dfs(mid)){
            r=mid-1;
            ans=mid;
        }
        else l=mid+1;
    }
    cout<<ans<<endl;

    return 0;
}