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  • CodeForces

    Description

    vjudge 上有中文题面。

    Solution

    挺好的费用流。

    建图太难描述了...我又懒得画图...看代码吧...

    #include<bits/stdc++.h>
    using namespace std;
    
    template <class T> void read(T &x) {
    	x = 0; bool flag = 0; char ch = getchar(); for (; ch < '0' || ch > '9'; ch = getchar()) flag |= ch == '-';
    	for (; ch >= '0' && ch <= '9'; ch = getchar()) x = x * 10 + ch - 48; flag ? x = ~x + 1 : 0;
    }
    
    #define N 1500
    #define rep(i, a, b) for (int i = (a); i <= (b); i++)
    #define INF 0x3f3f3f3f
    
    int S, T, flow, cost, head[N], tot = 1, dis[N], pre[N];
    struct edge { int u, v, c, w, next; }e[200001];
    queue<int> q;
    bool inq[N];
    inline void insert(int u, int v, int c, int w) { e[++tot].u = u, e[tot].v = v, e[tot].c = c, e[tot].w = w, e[tot].next = head[u], head[u] = tot; }
    inline void add(int u, int v, int c, int w) { insert(u, v, c, w), insert(v, u, 0, -w); }
    bool spfa() {
    	rep(i, S, T) dis[i] = INF; dis[S] = 0; q.push(S);
    	while (!q.empty()) {
    		int u = q.front(); q.pop(); inq[u] = 0;
    		for (int i = head[u], v, w; i; i = e[i].next) if (e[i].c > 0 && dis[v = e[i].v] > dis[u] + (w = e[i].w)) {
    			dis[v] = dis[u] + w, pre[v] = i;
    			if (!inq[v]) q.push(v); inq[v] = 1;
    		}
    	}
    	return dis[T] != INF;
    }
    void mcf() {
    	int d = INF;
    	for (int i = T; (i ^ S); i = e[pre[i]].u) d = min(d, e[pre[i]].c);
    	flow += d;
    	for (int i = T; (i ^ S); i = e[pre[i]].u) e[pre[i]].c -= d, e[pre[i] ^ 1].c += d, cost += d * e[pre[i]].w;
    }
    
    char s[N];
    int b[N], cnt[27];
    
    int main() {
    	int n; read(n);
    	scanf("%s", s + 1);
    	rep(i, 1, n) read(b[i]);
    	rep(i, 1, n) cnt[s[i] - 'a' + 1]++;
    	T = 26 + n + 26 * n / 2 + 1;
    	rep(i, 1, 26) {
    		add(S, i, cnt[i], 0);
    		rep(j, 1, n / 2) {
    			int t = 26 + n + (i - 1) * n / 2 + j, d1 = j, d2 = n - j + 1;
    			add(i, t, 1, 0);
    			add(t, d1 + 26, 1, -b[d1] * (s[d1] - 'a' + 1 == i));
    			add(t, d2 + 26, 1, -b[d2] * (s[d2] - 'a' + 1 == i));
    		}
    	}
    	rep(i, 1, n) add(i + 26, T, 1, 0);
    	while (spfa()) mcf();
    	printf("%d", -cost);
    	return 0;
    }
    
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  • 原文地址:https://www.cnblogs.com/aziint/p/9580798.html
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