SPOJ 1771 Yet Another N-Queen Problem

SPOJ_1771

    通过这个题目进一步训练了Dancing Links解决N皇后问题的技巧。

#include<stdio.h>
#include<string.h>
#include<stdlib.h>
#define INF 1000000000
const int MAXN = 50;
const int MAXmn = MAXN * MAXN * 4 + MAXN * 6 + MAXN;
const int MAXnn = MAXN * 6 + MAXN;
int N, size;
int U[MAXmn], D[MAXmn], L[MAXmn], R[MAXmn], H[MAXmn], C[MAXmn], X[MAXmn];
int Q[MAXnn], S[MAXnn], x[MAXnn], y[MAXnn], r[MAXnn], visc[MAXnn];
int cmp(const void *_p, const void *_q)
{
    int *p = (int *)_p;
    int *q = (int *)_q;
    return x[*p] - x[*q];
}
void prepare(int r, int c)
{
    int i, j;
    for(i = 0; i <= c; i ++)
    {
        S[i] = 0;
        U[i] = D[i] = i;
        R[i] = i + 1;
        L[i + 1] = i;
    }
    R[c] = 0;
    size = c;
    while(r)
        H[r --] = -1;
}
void place(int &r, int &c1, int &c2, int &c3, int &c4, int i, int j)
{
    r = (i - 1) * N + j;
    c1 = j;
    c2 = N + i;
    c3 = N * 3 + i - j;
    c4 = N * 4 + i + j - 2;
}
void link(int r,int c)
{
    size ++;
    C[size] = c;
    S[c] ++;
    X[size] = r;
    D[size] = D[c];
    U[D[c]] = size;
    U[size] = c;
    D[c] = size;
    if(H[r] < 0)
        H[r] = L[size] = R[size] = size;
    else
    {
        R[size] = R[H[r]];
        L[R[H[r]]] = size;
        L[size] = H[r];
        R[H[r]] = size;
    }
}
int init()
{
    int i, j, r, c1, c2, c3, c4;
    if(scanf("%d", &N) != 1)
        return 0;
    prepare(N * N, N * 6 - 2);
    memset(visc, 0, sizeof(visc));
    for(i = 1; i <= N; i ++)
    {
        scanf("%d", &j);
        if(j)
        {
            place(r, c1, c2, c3, c4, i, j);
            link(r, c1), link(r, c2), link(r, c3), link(r, c4);
            visc[c1] = visc[c2] = visc[c3] = visc[c4] = 1;
        }
    }
    for(i = 1; i <= N; i ++)
        for(j = 1; j <= N; j ++)
        {
            place(r, c1, c2, c3, c4, i, j);
            if(visc[c1] || visc[c2] || visc[c3] || visc[c4])
                continue;
            link(r, c1), link(r, c2), link(r, c3), link(r, c4);
        }
    return 1;
}
void remove(int c)
{
    int i, j;
    L[R[c]] = L[c];
    R[L[c]] = R[c];
    for(i = D[c]; i != c; i = D[i])
        for(j = R[i]; j != i; j = R[j])
        {
            U[D[j]] = U[j];
            D[U[j]] = D[j];
            S[C[j]] --;
        }
}
void resume(int c)
{
    int i, j;
    for(i = U[c]; i != c; i = U[i])
        for(j = L[i]; j != i; j = L[j])
        {
            S[C[j]] ++;
            U[D[j]] = j;
            D[U[j]] = j;
        }
    L[R[c]] = c;
    R[L[c]] = c;
}
int dance(int cur)
{
    int i, j, c, temp;
    if(cur >= N)
    {
        for(i = 0; i < cur; i ++)
        {
            x[i] = (X[Q[i]] - 1) / N + 1;
            y[i] = (X[Q[i]] - 1) % N + 1;
        }
        return 1;
    }
    if(!R[0])
        return 0;
    temp = INF;
    for(i = R[0]; i <= N; i = R[i])
        if(S[i] < temp)
        {
            temp = S[i];
            c = i;
        }
    remove(c);
    for(i = D[c]; i != c; i = D[i])
    {
        Q[cur] = i;
        for(j = R[i]; j != i; j = R[j])
        {
            remove(C[j]);
            }
        if(dance(cur + 1))
            return 1;
        for(j = L[i]; j != i; j = L[j])
            resume(C[j]);
    }
    resume(c);
    return 0;
}
void printresult()
{
    int i;
    for(i = 0; i < N; i ++)
        r[i] = i;
    qsort(r, N, sizeof(r[0]), cmp);
    for(i = 0; i < N; i ++)
    {
        if(i)
            printf(" ");
        printf("%d", y[r[i]]);
    }
    printf("\n");
}
int main()
{
    while(init())
    {
        if(dance(0))
            printresult();
    }
    return 0;
}