CF919D Substring

思路：

拓扑排序过程中dp。若图有环，返回-1。

实现：

#include <bits/stdc++.h>
using namespace std;
const int MAXN = 300005;
vector<int> G[MAXN];
int in[MAXN], n, m;
string value;
int dp[MAXN][26];
bool solve()
{
    queue<int> q;
    for (int i = 1; i <= n; i++) 
    {
        if (!in[i]) 
        {
            q.push(i); dp[i][value[i - 1] - 'a'] = 1;
        }
    }
    while (!q.empty())
    {
        int x = q.front(); q.pop();
        for (int i = 0; i < G[x].size(); i++)
        {
            int to = G[x][i];
            for (int k = 0; k < 26; k++) 
            {    
                if (k == value[to - 1] - 'a') dp[to][k] = max(dp[to][k], dp[x][k] + 1);
                else dp[to][k] = max(dp[to][k], dp[x][k]);
            }
            in[to]--;
            if (!in[to]) q.push(to);
        }
    }
    for (int i = 1; i <= n; i++) if (in[i]) return false;
    return true;
}
int main()
{
    int x, y;
    cin >> n >> m >> value;
    set<pair<int, int>> s;
    bool flg = true;
    for (int i = 0; i < m; i++) 
    { 
        cin >> x >> y;
        if (x == y) flg = false;
        pair<int, int> p(x, y);
        s.insert(p);
    }
    if (!flg) { cout << -1 << endl; return 0; }
    for (auto p: s) { G[p.first].push_back(p.second); in[p.second]++; }
    if (!solve()) cout << -1 << endl;
    else
    {
        int maxn = 0;
        for (int i = 1; i <= n; i++)
        {
            for (int j = 0; j < 26; j++)
                maxn = max(maxn, dp[i][j]);
        }
        cout << maxn << endl;
    }
    return 0;
}