[徐州网络赛]Longest subsequence

[徐州网络赛]Longest subsequence

可以分成两个部分，前面相同，然后下一个字符比对应位置上的大。

枚举这个位置

用序列自动机进行s字符串的下标转移

注意最后一个字符

#include <bits/stdc++.h>

const int maxn = 1e6 + 7;
using namespace std;
#define ll long long
int n, m;
char s[maxn], t[maxn];
int nex[maxn][27];

void init() {
    for (int k = 0; k < 26; ++k) nex[n][k] = -1;
    for (int i = n; i >= 1; --i) {
        for (int k = 0; k < 26; ++k) {
            nex[i - 1][k] = nex[i][k];
        }
        nex[i - 1][s[i] - 'a'] = i;
    }
}

int main() {
    ios::sync_with_stdio(false);
    cin.tie(0);
    cin >> n >> m;
    cin >> s + 1 >> t + 1;
    int minn;
    int ans = 0;
    int i, j;
    init();
    for (j = 0, i = 0; j <= m; ++j) {
        minn = 1e9;
        for (int k = max(0, t[j + 1] - 'a' + 1); k < 26; ++k) {
            if (nex[i][k] != -1) {
                minn = min(nex[i][k], minn);
            }
        }
        if (minn != 1e9) {
            ans = max(ans, n - minn + 1 + j);
        }
        if (j == m) break;
        i = nex[i][t[j + 1] - 'a'];
        if (i == -1) {
            break;
        }
    }
    if (ans == 0) {
        cout << -1 << endl;
    } else {
        cout << ans << endl;
    }
    return 0;
}