# Codeforces 919 D. Substring （记忆化搜索）

## Description

You are given a graph with n nodes and m directed edges. One lowercase letter is assigned to each node. We define a path’s value as the number of the most frequently occurring letter. For example, if letters on a path are “abaca”, then the value of that path is 3. Your task is find a path whose value is the largest.

## Input

The first line contains two positive integers n, m (1 ≤ n, m ≤ 300 000), denoting that the graph has n nodes and m directed edges.

The second line contains a string s with only lowercase English letters. The i-th character is the letter assigned to the i-th node.

Then m lines follow. Each line contains two integers x, y (1 ≤ x, y ≤ n), describing a directed edge from x to y. Note that x can be equal to y and there can be multiple edges between x and y. Also the graph can be not connected.

## Output

Output a single line with a single integer denoting the largest value. If the value can be arbitrarily large, output -1 instead.

## Examples input

5 4
abaca
1 2
1 3
3 4
4 5


## Examples output

3


## 思路

tarjan 判环，注意存在自环情况。

## AC 代码

#include <bits/stdc++.h>
#define IO ios::sync_with_stdio(false);\
cin.tie(0);\
cout.tie(0);
using namespace std;
const int maxn = 3e5+10;
const int mod = 1e9+7;
typedef __int64 LL;
#define inf 0x7f7f7f

struct node
{
int to;
int next;
} edge[maxn];
int dp[maxn][26],idx;
int ch[maxn],in[maxn];
bool instack[maxn];
int Stack[maxn],top;
int n,m;
bool flag;
char str[maxn];
void init()
{
memset(dp,0,sizeof dp);
memset(dfn,0,sizeof dfn);
memset(low,0,sizeof low);
memset(instack,false,sizeof instack);
top = idx = tot = 0;
flag = true;
}

{
edge[tot].to = v;
}

void dfs(int x)
{
dfn[x] = low[x] = ++idx;
instack[x] = true;
Stack[top++] = x;
{
int to = edge[i].to;
if(!dfn[to])
{
dfs(to);
low[x] = min(low[x],low[to]);
}
else if(instack[to] && dfn[to]<low[x])
low[x] = dfn[to];
for(int j=0; j<26; j++)
dp[x][j] = max(dp[x][j],dp[to][j]);
}
if(dfn[x]==low[x])
{
int Size = 0,now;
do
{
now = Stack[--top];
instack[now] = false;
++Size;
}
while(now!=x);
if(Size>1)
flag = false;
}
if(!flag)
return;
++dp[x][ch[x]];
}

void solve()
{
for(int i=1; i<=n; i++)
ch[i] = str[i-1] - 'a';
int ans = -1;
for(int i=1; i<=n; i++)
if(in[i]==0)
{
dfs(i);
for(int j=0; j<26; j++)
ans = max(ans,dp[i][j]);
}
cout<<(flag?ans:-1)<<endl;
}

int main()
{
IO;
init();
cin>>n>>m;
cin>>str;
for(int i=0; i<m; i++)
{
int u,v;
cin>>u>>v;
if(u==v)
flag = false;
++in[v];