hdu 1496

题目分类里说这题是hash，我用深搜加剪枝水过~~不过这道题还真算得上一道好题，思路很多，可以用HASH，题目数据再加强一点就更好了

深搜代码如下：

/*
 * hdu1496/linux.c
 * Created on: 2011-8-5
 * Author    : ben
 */
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <math.h>

void work();
int main() {
#ifndef ONLINE_JUDGE
    freopen("data.in", "r", stdin);
#endif
    work();
    return 0;
}

int a[4], ans, x[4];

void dfs(int step) {
    int i, temp;
    x[step] = 0;
    while (x[step] < 100) {
        x[step]++;
        if (step < 2) {
            dfs(step + 1);
        } else {
            temp = 0;
            for (i = 0; i < 3; i++) {
                temp += a[i] * x[i] * x[i];
            }
            if (temp <= 0 || temp + a[3] * 10000 > 0) {
                continue;
            }
            x[3] = sqrt(temp / (-a[3]));
            if (x[3] <= 100 && x[3] * x[3] * a[3] + temp == 0) {
                ans++;
            }
        }
    }
}

void work() {
    int i, j;
    while (scanf("%d%d%d%d", &a[0], &a[1], &a[2], &a[3]) == 4) {
        for (i = 0; i < 4; i++) {
            for (j = i + 1; j < 4; j++) {
                if (a[i] < a[j]) {
                    a[i] = a[i] ^ a[j];
                    a[j] = a[i] ^ a[j];
                    a[i] = a[i] ^ a[j];
                }
            }
        }
        if (a[3] > 0 || a[0] < 0) {
            puts("0");
            continue;
        }
        ans = 0;
        dfs(0);
        printf("%d\n", ans * 16);
    }
}

HASH方法代码：

/*
 * hdu1496/linux.c
 * Created on: 2011-8-5
 * Author    : ben
 */
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <math.h>
void work();
int main() {
#ifndef ONLINE_JUDGE
    freopen("data.in", "r", stdin);
#endif
    work();
    return 0;
}

int hash[2000010];
void work() {
    int a, b, c, d, i, j, count;
    while (scanf("%d %d %d %d", &a, &b, &c, &d) != EOF) {
        if ((a > 0 && b > 0 && c > 0 && d > 0)
                || (a < 0 && b < 0 && c < 0 && d < 0)) {
            puts("0");
            continue;
        }
        memset(hash, 0, sizeof(hash));
        count = 0;
        for (i = 1; i <= 100; i++) {
            for (j = 1; j <= 100; j++) {
                hash[a * i * i + b * j * j + 1000000]++;
            }
        }
        for (i = 1; i <= 100; i++) {
            for (j = 1; j <= 100; j++) {
                count += hash[1000000 - c * i * i - d * j * j];
            }
        }
        printf("%d\n", 16 * count);
    }
}