zoj 3732 Graph Reconstruction（构造）

题目链接：zoj 3732 Graph Reconstruction

解题思路

用贪心的方式取构造即可。

代码

/* hdu4797 */
#include <cstdio>
#include <cstring>
#include <algorithm>

using namespace std;
typedef pair<int,int> pii;
const int maxn = 105;

int N, E, ans, mv, P[maxn], C[maxn];
pii S[maxn*maxn], R[2][maxn*maxn];

bool cmp(const int& x, const int& y) {
    if (C[x] == C[y]) return x < y;
    return C[x] > C[y];
}

void dfs (int d) {
    sort(P + d, P + N, cmp);

    int rec = mv, rep = -1, u = P[d];


    if (d >= N || C[u] == 0) {
        for (int i = 0; i < mv; i++)
            R[ans][i] = S[i];
        ans++;
        return;
    }

    if (d + C[u] >= N) return;
    int ed = P[d+C[u]];
    if (C[ed] == 0) return;

    if (d+C[u]+1 < N) {
        rep = P[d+C[u]+1];
        if (C[ed] != C[rep]) rep = -1;
    }

    for (int i = 1; i <= C[u]; i++) {
        S[mv++] = make_pair(u, P[d+i]);
        C[P[d+i]]--;
    }
    dfs(d + 1);

    if (ans < 2 && rep != -1) {
        int v = S[--mv].second;
        C[v]++;
        S[mv++] = make_pair(u, rep);
        C[rep]--;

        dfs(d + 1);
    }

    while (mv > rec) {
        int v = S[--mv].second;
        C[v]++;
    }
}

int main () {
    while (scanf("%d", &N) == 1) {
        E = ans = mv = 0;
        for (int i = 0; i < N; i++) {
            scanf("%d", &C[i]);
            P[i] = i;
            E += C[i];
        }

        if ((E&1) == 0) dfs(0);

        E >>= 1;
        if (ans == 1) printf("UNIQUE\n");
        else if (ans == 2) printf("MULTIPLE\n");
        else  printf("IMPOSSIBLE\n");

        for (int i = 0; i < ans; i++) {
            printf("%d %d\n", N, E);

            for (int j = 0; j < E; j++) {
                if (j) printf(" ");
                printf("%d", R[i][j].first+1);
            }
            printf("\n");

            for (int j = 0; j < E; j++)  {
                if (j) printf(" ");
                printf("%d", R[i][j].second+1);
            }
            printf("\n");
        }

    }
    return 0;
}