电路图题解

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;
}

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;
}

三.电路图C：（全网首发）

大概题意：给你一张图，用k种颜色给图染色，保证相邻两个顶点不染同一种颜色，求染色方案数的表达式。

乍一看这道题可能无从下手，但经过简单的计算（可以试试数学课上排列组合的题目）发现最特别的情况就是几个点涂相同颜色的情况，那么就从全集中枚举涂相同颜色的集合，若最后枚举出来有$p$个集合，就会对答案产生$k(k-1)(k-2)……(k-p+1)$的贡献($p$个集合之间颜色互不相同)，当然在枚举集合的时候要判断集合之间的点是否有连边。

这样，你就得到了$40$分！

#include <iostream>
#include <cstdio>
#include <cstring>
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 SIZE = 18;
struct poly{
    int conf[SIZE+1];
    poly(){memset(conf, 0, sizeof(conf));}
    poly operator * (poly x){
        poly ans;
        for(int i=0; i<=SIZE; i++)
            for(int j=0; j<=SIZE-i; j++)
                ans.conf[i+j] += conf[i] * x.conf[j];
        return ans;
    }
    poly operator + (poly x){
        poly ans;
        for(int i=0; i<=SIZE; i++)
            ans.conf[i] = conf[i] + x.conf[i];
        return ans;
    }
};

const int MAXN = 19;
poly fac[MAXN], ans;
int g[MAXN][MAXN], n, m;

bool judge(int vis){
    for(int i=0; i<n; i++)
        for(int j=0; j<n; j++)
            if((vis & (1<<i)) && (vis & (1<<j)) && g[i][j])
                return false;
    return true;
}

void dfs(int vis,int deep){
    if(!vis){
        ans = ans + fac[deep];
        return;
    }
    int x = vis & (-vis);
    vis ^= x;
    for(int s=vis; ; s=(s-1)&vis){
        if(judge(s^x))
            dfs(vis^s, deep+1);
        if(!s)
            break;
    }
}

int main(){
    n = read(), m = read();
    for(int i=1; i<=n; i++){
        fac[i].conf[1] = 1;
        fac[i].conf[0] = 1-i;
        if(i != 1) fac[i] = fac[i-1] * fac[i];
    }
    for(int i=1; i<=m; i++){
        int x = read(), y = read();
        g[x][y] = g[y][x] = true;
    }
    dfs((1<<n)-1, 0);
    int len = SIZE;
    while(ans.conf[len] == 0)
        len--;
    for(int i=0; i<=len; i++)
        printf("%d
", ans.conf[i]);
    return 0;
}

因为一看数据规模就是成倍增长的，也就是说如果你优化了$O(n)$的复杂度你就可以多过3,4个点，首先当你枚举完时不要马上加到$ans$中，其次用状态压缩辅助判断集合是否合法。

期望得分:100，实际得分:70

#include <iostream>
#include <cstdio>
#include <cstring>
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 SIZE = 18;
struct poly{
    int conf[SIZE+1];
    poly(){memset(conf, 0, sizeof(conf));}
    poly operator * (poly x){
        poly ans;
        for(int i=0; i<=SIZE; i++)
            for(int j=0; j<=SIZE-i; j++)
                ans.conf[i+j] += conf[i] * x.conf[j];
        return ans;
    }
    poly operator + (poly x){
        poly ans;
        for(int i=0; i<=SIZE; i++)
            ans.conf[i] = conf[i] + x.conf[i];
        return ans;
    }
};

const int MAXN = 19;
poly fac[MAXN], ans;
int g[MAXN], n, m, cnt[MAXN];
int valid[1<<(MAXN-1)];

bool judge(int vis){
    if(valid[vis] != -1)
        return valid[vis];
    for(int i=0; i<n; i++)
        if((vis & (1<<i)) && (vis & g[i]))
            return valid[vis] = false;
    return valid[vis] = true;
}

void dfs(int vis,int deep){
    if(!vis){cnt[deep]++; return;}
    int x = vis & (-vis);
    vis ^= x;
    for(int s=vis; ; s=(s-1)&vis){
        if(judge(s^x))
            dfs(vis^s, deep+1);
        if(!s)
            break;
    }
}

int main(){
    memset(valid, -1, sizeof(valid));
    n = read(), m = read();
    for(int i=1; i<=n; i++){
        fac[i].conf[1] = 1;
        fac[i].conf[0] = 1-i;
        if(i != 1) fac[i] = fac[i-1] * fac[i];
    }
    for(int i=1; i<=m; i++){
        int x = read(), y = read();
        g[x] |= (1<<y), g[y] |= (1<<x);
    }
    dfs((1<<n)-1, 0);
    poly t;
    for(int i=1; i<=n; i++){
        t.conf[0] = cnt[i];
        ans = ans + t * fac[i];
    }
    int len = SIZE;
    while(ans.conf[len] == 0)
        len--;
    for(int i=0; i<=len; i++)
        printf("%d
", ans.conf[i]);
    return 0;
}

为什么呢，我发现我时间复杂度的估计有误，因为在$dfs$的时候枚举到了重复的$vis$，导致最后答案并不是成倍增长。其次$dfs$其实也有重复，对于一个$vis$，可以将接下来的搜索对答案的贡献保存下来（想象$dfs(vis,1)$和$dfs(vis,2)$之间贡献相差了多少呢？其实就是一个$k$而已），所以可以用记忆化搜索，将vis对答案的贡献用多项式保存下来，转移的时候只需把系数都左移一位（十进制的）即可。

时间复杂度：$O(nB_n)->O(n2^n)$，$B_n$是贝尔数，就是斯特林数的一层的和，如果不用$dp$那么一个图理论能分成多少个集合时间复杂度就是多少。

#include <iostream>
#include <cstdio>
#include <cstring>
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 SIZE = 18;
struct poly{
    int conf[SIZE+1];
    poly(){memset(conf, 0, sizeof(conf));}
    poly operator * (poly x){
        poly ans;
        for(int i=0; i<=SIZE; i++)
            for(int j=0; j<=SIZE-i; j++)
                ans.conf[i+j] += conf[i] * x.conf[j];
        return ans;
    }
    poly operator + (poly x){
        poly ans;
        for(int i=0; i<=SIZE; i++)
            ans.conf[i] = conf[i] + x.conf[i];
        return ans;
    }
};

const int MAXN = 19;
poly fac[MAXN], ans, dp[1<<(MAXN-1)];
int g[MAXN], n, m;
int valid[1<<(MAXN-1)], isCount[1<<(MAXN-1)];

bool judge(int vis){
    if(valid[vis] != -1)
        return valid[vis];
    for(int i=0; i<n; i++)
        if((vis & (1<<i)) && (vis & g[i]))
            return valid[vis] = false;
    return valid[vis] = true;
}

poly dfs(int vis){
    if(isCount[vis]) return dp[vis];
    int x = vis & (-vis), rest = vis ^ x;
    poly cnt;
    for(int s=rest; ; s=(s-1)&rest){
        if(judge(s^x))
            cnt = cnt + dfs(rest^s);
        if(!s) break;
    }
    for(int i=SIZE; i>=1; i--)
        cnt.conf[i] = cnt.conf[i-1];
    cnt.conf[0] = 0, isCount[vis] = true;
    return dp[vis] = cnt;
}

int main(){
    memset(valid, -1, sizeof(valid));
    n = read(), m = read();
    for(int i=1; i<=n; i++){
        fac[i].conf[1] = 1;
        fac[i].conf[0] = 1-i;
        if(i != 1) fac[i] = fac[i-1] * fac[i];
    }
    for(int i=1; i<=m; i++){
        int x = read(), y = read();
        g[x] |= (1<<y), g[y] |= (1<<x);
    }
    poly t, cnt;
    isCount[0] = true, dp[0].conf[0] = 1;
    cnt = dfs((1<<n)-1);
    for(int i=1; i<=n; i++){
        t.conf[0] = cnt.conf[i];
        ans = ans + t * fac[i];
    }
    int len = SIZE;
    while(ans.conf[len] == 0)
        len--;
    for(int i=0; i<=len; i++)
        printf("%d
", ans.conf[i]);
    return 0;
}