Anti-Palindromize

想到網絡流就差不多了， 拆拆點， 建建邊。

#include<bits/stdc++.h>
#define LL long long
#define fi first
#define se second
#define mk make_pair
#define PLL pair<LL, LL>
#define PLI pair<LL, int>
#define PII pair<int, int>
#define SZ(x) ((int)x.size())
#define ull unsigned long long

using
 
 namespace std;

const int N = 1500 + 7;
const int inf = 0x3f3f3f3f;
const LL INF = 0x3f3f3f3f3f3f3f3f;
const int mod = 1e9 + 7;
const double eps = 1e-9;
const double PI = acos(-1);

int edgenum, S, T;
int head[N], dist[N], pre[N];
bool vis[N];

struct Edge {
    int from, to, cap, flow, cost, next;
} edge[
 
1000007];

void MCMFinit() {
    edgenum = 0;
    memset(head, -1, sizeof(head));
}

void addEdge(int u, int v, int w, int c) {
    Edge E1 = {u, v, w, 0, c, head[u]};
    edge[edgenum] = E1;
    head[u] = edgenum++;
    Edge E2 = {v, u, 0, 0, -c, head[v]};
    edge[edgenum] = E2;
    head[v] = edgenum++;
}

 
bool SPFA(int s, int t) {
    queue<int> Q;
    memset(dist, INF, sizeof(dist));
    memset(vis, false, sizeof(vis));
    memset(pre, -1, sizeof(pre));
    dist[s] = 0; vis[s] = true; Q.push(s);
    while(!Q.empty()) {
        int u = Q.front(); Q.pop(); vis[u] = false;
        for(int i = head[u]; i != -1; i = edge[i].next) {
            Edge E = edge[i];
            if(dist[E.to] > dist[u] + E.cost && E.cap > E.flow) {
                dist[E.to] = dist[u] + E.cost;
                pre[E.to] = i;
                if(!vis[E.to]) {
                    vis[E.to] = true;
                    Q.push(E.to);
                }
            }
        }
    }
    return pre[t] != -1;
}

void MCMF(int s, int t, int &cost, int &flow) {
    flow = 0; cost = 0;
    while(SPFA(s, t)) {
        int Min = INF;
        for(int i = pre[t]; i != -1; i = pre[edge[i^1].to]) {
            Edge E = edge[i];
            Min = min(Min, E.cap - E.flow);
        }
        for(int i = pre[t]; i != -1; i = pre[edge[i^1].to]) {
            edge[i].flow += Min;
            edge[i^1].flow -= Min;
            cost += edge[i].cost * Min;
        }
        flow += Min;
    }
}

int n;
char s[N];
int a[N], b[N];
int cnt[100];

int main() {
    MCMFinit();
    scanf("%d", &n);
    S = 0, T = (n / 2 + 1) * 26 + n + 1;
    int B = (n / 2 + 1);
    int G = (n / 2 + 1) * 26;
    scanf("%s", s + 1);
    for(int i = 1; i <= n; i++) scanf("%d", &b[i]);
    for(int i = 1; i <= n; i++) {
        a[i] = s[i] - ‘a‘ + 1;
        cnt[a[i]]++;
    }
    for(int i = 1, o = 1; i <= G; i += B, o++) {
        addEdge(S, i, cnt[o], 0);
        for(int j = 1; j < B; j++) {
            addEdge(i, i + j, 1, 0);
            int c1 = a[j] == o ? -b[j] : 0;
            int c2 = a[n - j + 1] == o ? -b[n - j + 1] : 0;
            addEdge(i + j, G + j, 1, c1);
            addEdge(i + j, G + n - j + 1, 1, c2);
        }
    }
    for(int i = 1; i <= n; i++) addEdge(G + i, T, 1, 0);
    int cost = 0, flow = 0;
    MCMF(S, T, cost, flow);
    printf("%d\n", -cost);
    return 0;
}

/*
*/

Codeforces 844F Anti-Palindromize 最小費用流