[CF762C] Two strings - 双指针

[CF762C] Two strings - 双指针

Description

给定 \(2\) 仅包含小写字母字符串 \(s\) 和 \(t\)，从 \(s\) 中删除连续的一段字符，使得 \(s\) 是 \(t\) 的子序列。最小化删除的数量，并输出执行删除操作后的字符串 \(s\)。

Solution

预处理 \(s\) 的每个前缀在 \(t\) 中从头开始做子序列匹配到的位置，\(s\) 的每个后缀在 \(t\) 中从尾反向开始做子序列匹配到的位置，然后双指针处理即可

#include <bits/stdc++.h>
using namespace std;

#define int long long

const int N = 1e6 + 5;

char s[N], t[N];
int a[N], b[N], n, m;

signed main()
{
    ios::sync_with_stdio(false);

    cin >> t + 1 >> s + 1;
    n = strlen(s + 1);
    m = strlen(t + 1);

    for (int i = 1; i <= n; i++)
    {
        int j;
        for (j = a[i - 1] + 1; j <= m; j++)
            if (s[i] == t[j])
                break;
        if (j <= m)
            a[i] = j;
        else
            a[i] = m + 1;
    }

    b[n + 1] = m + 1;
    for (int i = n; i >= 1; i--)
    {
        int j;
        for (j = b[i + 1] - 1; j >= 1; j--)
            if (s[i] == t[j])
                break;
        if (j >= 1)
            b[i] = j;
        else
            b[i] = 0;
    }

    int ans = 0, ansx, ansy;
    for (int i = 1, j = 2; i <= n; i++)
    {
        if (i >= j)
            j = i + 1;
        while (a[i] >= b[j])
            j++;
        if (i + n - j + 1 > ans)
        {
            ans = i + n - j + 1;
            ansx = i;
            ansy = j;
        }
    }

    for (int i = 1; i <= n; i++)
    {
        if (a[i] <= m && i > ans)
        {
            ans = i;
            ansx = i;
            ansy = n + 1;
        }
    }

    for (int i = 1; i <= n; i++)
    {
        if (b[i] >= 1 && n - i + 1 > ans)
        {
            ans = n - i + 1;
            ansx = 0;
            ansy = i;
        }
    }

    // for (int i = 1; i <= n; i++)
    //     cout << a[i] << " ";
    // cout << endl;
    // for (int i = 1; i <= n; i++)
    //     cout << b[i] << " ";
    // cout << endl;

    if (ans == 0)
    {
        cout << "-" << endl;
    }
    else
    {
        for (int i = 1; i <= ansx; i++)
            cout << s[i];
        for (int j = ansy; j <= n; j++)
            cout << s[j];
        cout << endl;
    }
}