该程序的最坏时间复杂度为O(n×m)。（ ）

#include <bits/stdc++.h>
using namespace std;
int n, m, s, e, head[105], to[205], nxt[205];
bool v[100005];
vector <int> a;
void dfs(int u) {
    v[u] = 1;
    a.push_back(u);
    if (u == e) {
        for (int i = 0; i < a.size(); i++)
            printf("%d ", a[i]);
        printf("\n");
        v[u] = 0;
        a.pop_back();
        return;
    }
    for (int i = head[u]; i; i = nxt[i])
        if (!v[to[i]]) dfs(to[i]);
    v[u] = 0;
    a.pop_back();
}

int main() {
    scanf("%d%d", &n, &m);
    for (int i = 1; i <= m; i++) {
        int u, v;
        scanf("%d%d", &u, &v);
        to[i] = v;
        nxt[i] = head[u];
        head[u] = i;
    }
    scanf("%d%d", &s, &e);
    dfs(s);
    return 0;
}