洛谷P4716 【模板】最小树形图(朱刘算法)

题意

题目链接

sol

朱刘算法？感觉又是一种神仙贪心算法

大概就是每次贪心的用每个点边权最小的入边更新答案，如果不行的话就缩起来找其他的边

不详细说了，丢走人..

#include<bits/stdc++.h>
using namespace std;
const int maxn = 1e5 + 10, inf = 1e9 + 10;
inline int read() {
    char c = getchar(); int x = 0, f = 1;
    while(c < '0' || c > '9') {if(c == '-') f = -1; c = getchar();}
    while(c >= '0' && c <= '9') x = x * 10 + c - '0', c = getchar();
    return x * f;
}
int n, m, r, fa[maxn], mn[maxn], id[maxn], vis[maxn];
struct edge {
    int u, v, w, nxt;
}e[maxn];
int head[maxn], num = 1;
inline void addedge(int x, int y, int z) {
    e[num] = (edge) {x, y, z, head[x]}; head[x] = num++;
}
int zhuliu() {
    int ans = 0;
    while("attack is a pig") {
        for(int i = 1; i <= n; i++) id[i] = vis[i] = 0, mn[i] = inf; int cnt = 0;
        for(int i = 1; i <= m; i++) if((e[i].u != e[i].v) && (e[i].w < mn[e[i].v])) mn[e[i].v] = e[i].w, fa[e[i].v] = e[i].u;
        int x; mn[r] = 0;//tag 
        for(int i = 1; i <= n; i++) {
            if(mn[i] == inf) return -1; ans += mn[i];
            for(x = i; (!id[x]) && x != r && (vis[x] != i); x = fa[x]) vis[x] = i;
            if(x != r && (!id[x])) {
                id[x] = ++cnt; for(int t = fa[x]; t != x; t = fa[t]) id[t] = cnt;
            }
        }
        if(!cnt) return ans;
        for(int i = 1; i <= n; i++) if(!id[i]) id[i] = ++cnt;
        for(int i = 1; i <= m; i++) {
            int tmp = mn[e[i].v];
            if((e[i].u = id[e[i].u]) != (e[i].v = id[e[i].v])) e[i].w -= tmp;
        }
        n = cnt; r = id[r]; 
    }
    return ans;
}
int main() {
    memset(head, -1, sizeof(head));
    n = read(); m = read(); r = read();
    for(int i = 1; i <= m; i++) {
        int x = read(), y = read(), z = read();
        addedge(x, y, z);
    }
    printf("%d", zhuliu());
    return 0;
}