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，丢链接走人..

#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;
}