competitive-programming/solved-uva/11686.cpp at master · the-redback/competitive-programming · GitHub

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/**
 *                                   ____
 *       ____ ___  ____ ________  __/ __/
 *      / __ `__ \/ __ `/ ___/ / / / /_
 *     / / / / / / /_/ / /  / /_/ / __/
 *    /_/ /_/ /_/\__,_/_/   \__,_/_/
 *
 *    @link : https://the-redback.com
 */
#include <bits/stdc++.h>
using namespace std;

#define mem(a, b) memset(a, b, sizeof(a))
#define pb        push_back
#define pp        pop_back
#define inf       1000000000
#define NN        1000010

vector<int> e[NN + 7], v;
int view[NN + 7];
int f, fl;

void dfs(int u) {
    int i, k, l;
    view[u] = 0;
    for (i = 0; i < e[u].size(); i++) {
        if (view[e[u][i]] == -1)
            dfs(e[u][i]);
        else if (view[e[u][i]] == 0) {
            fl = 1;
            return;
        }
    }
    view[u] = 1;
    v.pb(u);
}

int main() {
    int i, j, k, l;
    int tc, t;
    int n, m;
    while (~scanf("%d%d", &n, &m)) {
        if (n == 0 && m == 0) return 0;
        mem(view, -1);
        for (i = 0; i < m; i++) scanf("%d%d", &k, &l), e[k].pb(l);
        fl = 0;
        for (i = 1; i <= n; i++) {
            f = i;
            if (view[i] == -1) dfs(i);
        }
        if (fl)
            printf("IMPOSSIBLE\n");
        else {
            reverse(v.begin(), v.end());
            for (i = 0; i < v.size(); i++) printf("%d\n", v[i]);
        }
        v.clear();
        for (i = 0; i <= n; i++) e[i].clear();
    }
    return 0;
}