电路图题解

Posted 2021-03-11 phdhd

OJ 1177 1178 1179

一.电路图A

第一问，容易看出$右拐次数=左拐次数+4$，$左拐+右拐=n$，所以$右拐=n/2-2$，相当于$C_{n}^{n/2-2}$

第二问，总个数除去最左端，最右端，最上方，最下方的电阻，整个电路被分成$4$段，每一段都有偶数个电阻，答案就是将$n$分成$4$个偶数的情况，相当于将$n/2$分成任意数的情况，用隔板法，答案就是$C_{n/2-1}^{3}$，排除旋转的影响，答案还要除以$4$。

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

inline long long read(){
    long long ans = 0, f = 1;
    char ch = getchar();
    while(!isdigit(ch))
        f *= (ch == ‘-‘) ? -1 : 1, ch = getchar();
    do ans = (ans << 1) + (ans << 3) + (ch ^ 48), ch = getchar();
    while(isdigit(ch));
    return ans * f;
}

const int MOD = 1e9+7, MAXN = 1e7 + 1;
int fac[MAXN], inv[MAXN];

int pow(int a,int p){
    int ans = 1;
    while(p){
        if(p & 1) ans = (long long)ans * a % MOD;
        a = (long long)a * a % MOD, p >>= 1;
    }
    return ans;
}

inline int C(int n,int m){
    return (long long)fac[n] * inv[m] % MOD * inv[n-m] % MOD;
}

int main(){
    
    int n = read(), ans1, ans2;
    fac[0] = 1;
    for(int i=1; i<=n; i++)
        fac[i] = (long long)fac[i-1] * i % MOD;
    inv[n] = pow(fac[n], MOD-2);
    for(int i=n-1; i>=0; i--)
        inv[i] = (long long)inv[i+1] * (i+1) % MOD;
    ans1 = C(n, n/2-2);
    ans2 = (long long)C(n/2+1, 3) * n % MOD * pow(4, MOD-2) % MOD;
    printf("%d
%d", ans1, ans2);
    return 0;
}

View Code

二.电路图B

区间修改，区间查询，一看就是线段树，关键在于合并标记如何合并。

可将一个标记记为$frac{ax+b}{cx+d}$，用四个数表示为${a,b,c,d}$，串联一个电阻就是${0,R,0,1}$，并联一个电阻就是${R,0,1,R}$

最关键是mix操作，将${a,b,c,d}$和${i,j,k,p}$合成为${ai+cj,bi+dj,ak+cp,bk+dp}$，最终就转化为线段树加标记的问题了。

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

long long read(){
    long long ans = 0, f = 1;
    char ch = getchar();
    while(ch < ‘0‘ || ch > ‘9‘)
        f *= (ch == ‘-‘) ? -1 : 1, ch = getchar();
    do ans = ((ans << 1) + (ans << 3) + (ch ^ 48)), ch = getchar();
    while(ch >= ‘0‘ && ch <= ‘9‘);
    return ans * f;
}

const int MAXN = 250001, MAXM = 250001;

struct sign{double a, b, c, d;};

struct Segment{
    int l, r;
    double maxNum, minNum;
    sign add;
    bool isSigned;
}seg[MAXN*4];
int a[MAXN];

void change(int x,sign fa){
    sign t = seg[x].add;
    seg[x].isSigned = true;
    double a, b, c, d;
    a = t.a * fa.a + t.c * fa.b;
    b = t.b * fa.a + t.d * fa.b;
    c = t.a * fa.c + t.c * fa.d;
    d = t.b * fa.c + t.d * fa.d;
    seg[x].add = (sign){1, b/a, c/a, d/a};
    seg[x].maxNum = (seg[x].maxNum * fa.a + fa.b) / (seg[x].maxNum * fa.c + fa.d);
    seg[x].minNum = (seg[x].minNum * fa.a + fa.b) / (seg[x].minNum * fa.c + fa.d);
}

void spread(int x){
    if(seg[x].isSigned){
        change(x*2, seg[x].add);
        change(x*2+1, seg[x].add);
        seg[x].isSigned = false;
        seg[x].add = (sign){1, 0, 0, 1};
    }
}

void build(int l,int r,int x=1){
    seg[x].l = l, seg[x].r = r;
    seg[x].add = (sign){1, 0, 0, 1};
    if(l == r){
        seg[x].maxNum = seg[x].minNum = a[l];
        return;
    }
    int mid = (l + r) >> 1;
    build(l, mid, x*2);
    build(mid+1, r, x*2+1);
    seg[x].maxNum = max(seg[x*2].maxNum, seg[x*2+1].maxNum);
    seg[x].minNum = min(seg[x*2].minNum, seg[x*2+1].minNum);
}

void point(int l,int r,sign vis,int x=1){
    if(l <= seg[x].l && seg[x].r <= r){
        change(x, vis);
        return;
    }
    spread(x);
    if(seg[x*2].r >= l) point(l, r, vis, x*2);
    if(seg[x*2+1].l <= r) point(l, r, vis, x*2+1);
    seg[x].maxNum = max(seg[x*2].maxNum, seg[x*2+1].maxNum);
    seg[x].minNum = min(seg[x*2].minNum, seg[x*2+1].minNum);
}

double ask(int l,int r,int type,int x=1){
    if(l <= seg[x].l && seg[x].r <= r)
        return (type == 1) ? seg[x].maxNum : seg[x].minNum;
    spread(x);
    double ans = (type == 1) ? 0 : 1e9;
    if(seg[x*2].r >= l) (type == 1) ? ans = max(ans, ask(l, r, type, x*2)) : ans = min(ans, ask(l, r, type, x*2));
    if(seg[x*2+1].l <= r) (type == 1) ? ans = max(ans, ask(l, r, type, x*2+1)) : ans = min(ans, ask(l, r, type, x*2+1));
    return ans;
}

int main(){
    int n = read();
    for(int i=1; i<=n; i++)
        a[i] = read();
    build(1, n);
    int m = read();
    for(int i=1; i<=m; i++){
        int op = read(), l = read(), r = read();
        if(op == 1) printf("%.10lf
", ask(l, r, 1));
        else if(op == 2) printf("%.10lf
", ask(l, r, 2));
        else{
            int R = read();
            if(op == 3) point(l, r, (sign){1, R, 0, 1});
            else point(l, r, (sign){R, 0, 1, R});
        }
    }
    return 0;
}

View Code

三.电路图C（待填坑）