不仅要输出最长公共子序列的长度，还要输出路径

#include <iostream>
#include <algorithm>
#include <cstring>


using namespace std;

const int N = 1010;

int n, m;
char a[N], b[N];
int f[N][N];

int main()
{
    cin >> n >> m;
    cin >> a + 1 >> b + 1;

    for (int i = 1; i <= n; i ++)
        for (int j = 1; j <= m; j ++)
        {
            f[i][j] = max(f[i - 1][j], f[i][j - 1]);
            if (a[i] == b[j]) f[i][j] = max(f[i][j], f[i - 1][j - 1] + 1);
        }
    cout << f[n][m] << endl;

    string res;
    // 一个倒序的过程
    for (int i = n, j = m; i && j; )
    {
        if (a[i] == b[j]) res += a[i], i --, j --;
        else if (f[i - 1][j] > f[i][j - 1]) i --;
        else j --;
    }

    reverse(res.begin(), res.end());

    cout << res << endl;
    return 0;
}