bzoj 3251

http://www.lydsy.com/JudgeOnline/problem.php?id=3251

这道题在北京八十中的时候有人讲过。。 不过由于自己continue 写掉了一个所以调了很久。

做法是如果整个序列没有合法三角形的话，那么整个链长不超过50个（最大的情况是斐波那契数列） 所以大于50个一定成立， 小于50个排序扫一遍就好了

#include <iostream>
#include <cstdio>
#include <cstring>
#include <algorithm>
using namespace std;

typedef long long ll;

const ll maxn = 200010;
const ll maxv = 50;

ll int_get() {
    ll x = 0; char c = (char)getchar(); bool f = 0;
    while(!isdigit(c)) {
        if(c == '-') f = 1;
        c = (char)getchar();
    }
    while(isdigit(c)) {
        x = x * 10 + (ll)(c - '0');
        c = (char)getchar();
    }
    if(f) x = -x;
    return x;
}

struct edge {
    ll t; edge* next;
}e[maxn * 2], *head[maxn]; ll ne = 0;

void addedge(ll f, ll t) {
    e[ne].t = t, e[ne].next = head[f], head[f] = e + ne ++;
}

ll h[maxn];
ll n, m, v[maxn], fa[maxn];

void dfs(ll x, ll pre) {
    h[x] = h[pre] + 1;  fa[x] = pre;
    for(edge* p = head[x]; p; p = p-> next) {
        if(p-> t != pre) dfs(p-> t, x);
    }
}

void read() {
    n = int_get(); m = int_get();
    for(ll i = 1; i <= n; ++ i) v[i] = int_get();
    for(ll i = 1; i < n; ++ i) {
        ll f, t;
        f = int_get(), t = int_get();
        addedge(f, t), addedge(t, f);
    }
    dfs(1, 0);
}

ll sta[maxn], top = 0;

void sov() {
    while(m --) {
        ll opt = int_get();
        if(!opt) {
            ll a = int_get(), b = int_get();
            top = 0;
            if(h[a] < h[b]) swap(a, b);
            while(h[a] != h[b] && top <= maxv) sta[++ top] = v[a], a = fa[a];
            if(a != b) {
                while(a != b && top <= maxv) sta[++ top] = v[a], sta[++ top] = v[b], a = fa[a], b = fa[b];
            }
            sta[++ top] = v[a];
            if(top >= maxv) {
                printf("Y\n"); continue;
            }
            sort(sta + 1, sta + top + 1); bool f = 0;
            for(ll i = 1; i < top - 1 && !f; ++ i) {
                if(sta[i] + sta[i + 1] > sta[i + 2]) f = 1;
            }
            if(f) printf("Y\n");
            else printf("N\n");
        }
        else {
            ll p = int_get(), w = int_get();
            v[p] = w;
        }
    }
}

int main() {
    //freopen("test.in", "r", stdin);
    //freopen("test.out", "w", stdout);
    read(), sov();
    return 0;
}