ztr loves lucky numbers 傻逼的我来了个大模拟

http://acm.hdu.edu.cn/showproblem.php?pid=5676 

这题的正解因该是dfs的，但是有18个位，然后我一算，全排列的话，有18!个啊，那不是很大？但是有很多是相同的，因为4477和第一个和第二个数字调转的结果是一样的。

先说说我模拟的方法。

真的很麻烦，不想看的，给几组数据就跑

7
0
8
4500
47
55
44447778
78
47777445

我是贪心模拟前n位，模拟的时候，如果这一位是3，如果我还有4，那么证明比后面的大了，然后后面的直接按4优先输出即可。

还有可能要加位的，就是78这样，要加位。

#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cmath>
#include <algorithm>
#define IOS ios::sync_with_stdio(false)
using namespace std;
#define inf (0x3f3f3f3f)
typedef long long int LL;

#include <iostream>
#include <sstream>
#include <vector>
#include <set>
#include <map>
#include <queue>
#include <string>
const int maxn = 1e2 + 20;
char str[maxn];
char ans[maxn];
int findnotseven(int begin, int end) {
    for (int i = end; i >= begin; --i) {
        if (ans[i] != '7') return i;
    }
    return inf;
}
void work() {
    for (int i = 1; i <= 100; ++i) str[i] = '0';
    scanf("%s", str + 1);
    int lenstr = strlen(str + 1);
    bool big = false;
    int lenans = lenstr;
    int four;
    int seven;
    if (lenstr & 1) {
        big = true;
        four = lenstr / 2 + 1;
        seven = lenstr / 2 + 1;
        lenstr++;
    } else {
        four = lenstr / 2;
        seven = lenstr / 2;
    }
    int tnow = 0;
    int add = 0;
    int pos = false;
    char what;
    for (int i = 1; i <= lenstr; ++i) {
        if (big) {
            if (four) {
                ans[++tnow] = '4';
                four--;
            } else if (seven) {
                ans[++tnow] = '7';
                seven--;
            } else while(1);
        } else {
            if (str[i] > '7') {
                if (pos) {
                    ans[pos] = what;
                    four = 0;
                    seven = 0;
                    for (int k = 1; k <= pos; ++k) {
                        four += ans[k] == '4';
                        seven += ans[k] == '7';
                    }
                    four = lenstr / 2 - four;
                    seven = lenstr / 2 - seven;
                    big = true;
                    i = pos;
                    tnow = pos;
                } else {
                    add = 2;
                    big = true;
                    four = (lenstr + 2) / 2 - 2;
                    seven = (lenstr + 2) / 2;
                    i = 0;
                    tnow = 0;
                }
            } else {
                if (str[i] >= '5' && str[i] <= '7') {
                    if (seven) {
                        ans[++tnow] = '7';
                        seven--;
                        if (str[i] != '7') {
                            big = true;
                            four = 0;
                            seven = 0;
                            for (int k = 1; k <= i; ++k) {
                                four += ans[k] == '4';
                                seven += ans[k] == '7';
                            }
                            four = lenstr / 2 - four;
                            seven = lenstr / 2 - seven;
                            //************//
                            pos = i;
                            what = '7';
                        }
                    } else { //也要改
                        if (pos) {
                            ans[pos] = what;
                            four = 0;
                            seven = 0;
                            for (int k = 1; k <= pos; ++k) {
                                four += ans[k] == '4';
                                seven += ans[k] == '7';
                            }
                            four = lenstr / 2 - four;
                            seven = lenstr / 2 - seven;
                            big = true;
                            i = pos;
                            tnow = pos;
                        } else {
                            add = 2;
                            big = true;
                            four = (lenstr + 2) / 2 - 2;
                            seven = (lenstr + 2) / 2;
                            i = 0;
                            tnow = 0;
                        }
                    }
                } else {
                    if (four) {
                        ans[++tnow] = '4';
                        four--;
                        if (str[i] != '4') {
                            big = true;
                            four = 0;
                            seven = 0;
                            for (int k = 1; k <= i; ++k) {
                                four += ans[k] == '4';
                                seven += ans[k] == '7';
                            }
                            four = lenstr / 2 - four;
                            seven = lenstr / 2 - seven;
                            pos = i;
                            what = '4';
                        } else {
                            if (seven) {
                                pos = i;
                                what = '7';
                            }
                        }
                    } else if (seven) {
                        ans[++tnow] = '7';
                        seven--;
                        big = true;
                        four = 0;
                        seven = 0;
                        for (int k = 1; k <= i; ++k) {
                            four += ans[k] == '4';
                            seven += ans[k] == '7';
                        }
                        four = lenstr / 2 - four;
                        seven = lenstr / 2 - seven;
                        pos = i;
                        what = '4';
                        pos = i;
                        what = '7';
                    }
                }
            }
        }
    }
    while (add--) {
        printf("4");
    }
    for (int i = 1; i <= tnow; ++i) {
        printf("%c", ans[i]);
    }
    printf("
");
}

int main() {
#ifdef local
    freopen("data.txt","r",stdin);
#endif
    int t;
    scanf("%d", &t);
    while (t--) work();
    return 0;
}

说一下按位DFS的做法

预处理出所有结果，首先要知道有多少种结果

当数字有18位的时候，9位4,9位7， 那么C(18, 9)的意思就是选9个位置来放四，其他的放7，那么结果就是C（18,9）

所以总答案数只有66196种。

#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cmath>
#include <algorithm>
#define IOS ios::sync_with_stdio(false)
using namespace std;
#define inf (0x3f3f3f3f)
typedef long long int LL;

#include <iostream>
#include <sstream>
#include <vector>
#include <set>
#include <map>
#include <queue>
#include <string>
const int maxn = 1e6 + 20;
LL ans[maxn];
int lenans;
void dfs(LL now, int num_four, int num_seven) {
    if (num_four == 0 && num_seven == 0) {
        ans[++lenans] = now;
        return;
    }
    if (num_four == 0) {
        dfs(now * 10 + 7, num_four, num_seven - 1);
    } else if (num_seven == 0) {
        dfs(now * 10 + 4, num_four - 1, num_seven);
    } else {
        dfs(now * 10 + 4, num_four - 1, num_seven);
        dfs(now * 10 + 7, num_four, num_seven - 1);
    }
}
void init() {
    for (int i = 2; i <= 18; i += 2) {
        dfs(0, i / 2, i / 2);
    }
    cout << lenans << endl;
}
void work() {
    LL n;
    scanf("%I64d", &n);
    if (n > ans[lenans]) { //最大值777777777444444444。1e17内的最大值
        printf("44444444447777777777
");
    } else {
//        LL tans = lower_bound(ans + 1, ans + 1 + lenans);
//        printf("%I64d
", tans);
        int begin = 1, end = lenans;
        while (begin <= end) {
            int mid = (begin + end) >> 1;
            if (ans[mid] >= n) {
                end = mid - 1;
            } else begin = mid + 1;
        }
        printf("%I64d
", ans[begin]);
    }
}
int main() {
#ifdef local
    freopen("data.txt","r",stdin);
#endif
    init();
    int t;
    scanf("%d", &t);
    while (t--) work();
    return 0;
}

可以学习下DFS的做法