USACO 4.4 shuttle puzzle

起初用搜索，写了好多，bfs+位运算。。。然后STL的队列给MLE了。。。但是算出的结果是正确的，然后接着是TLE。。。

虽然用搜索没有挂，但是不得不说又练了一下代码能力，还是有不少收获的，呵呵。。

正确的算法是找规律，详细的做法看nocow吧，这里贴个代码：

搜索：

/*
ID: zlqest11
LANG: C++
TASK: shuttle
*/
#include <iostream>
#include <cstdio>
#include <cstring>
#include <set>
#include <vector>
#include <queue>
#include <utility>
using namespace std;

//file stream declare
FILE fin, fout;
//data structure
const int N = 1000000;
set < pair<short, int> > hash;
queue < pair<short, int> > q;
int n, start, target;
int head, tail, step[N], pre[N], mid[N];

vector < pair <short, int> > ans;

void mkbit(int &x, int pos, int flag){
    int tmp;
    if(flag == 0){
        tmp = 1<<pos;
        tmp = ~tmp;
        x &= tmp;
    }
    else{
        tmp = 1<<pos;
        x |= tmp;
    }
}

void b_bfs(){
    //init
    memset(step, -1, sizeof(step));
    memset(pre, -1, sizeof(pre));
    //search process
    pair <short, int> u, tmp;
    short pos;
    int key, xx;
    head = tail = 0;
    while(!q.empty()){
        u = q.front();
        q.pop();
        pos = u.first;
        key = u.second;
        if(pos < n*2){
            tmp.first = pos+1;
            tmp.second = key;
            if(hash.find(tmp) == hash.end()){
                hash.insert(tmp);
                q.push(tmp);
                step[++tail] = pos;
                pre[tail] = head;
                mid[tail] = pos;
                if(tmp.second==target && tmp.first==n)  return;
            }
        }
        if(pos >= 1){
            tmp.first = pos-1;
            tmp.second = key;
            if(hash.find(tmp) == hash.end()){
                hash.insert(tmp);
                q.push(tmp);
                step[++tail] = pos-1;
                pre[tail] = head;
                mid[tail] = pos;
                if(tmp.second==target && tmp.first==n)  return;
            }
        }
        if(n*2-pos >= 2){
            tmp.second = key;
            if(key&(1<<pos+1))  mkbit(tmp.second, pos, 1);
            else  mkbit(tmp.second, pos, 0);
            if(key&(1<<pos))  mkbit(tmp.second, pos+1, 1);
            else  mkbit(tmp.second, pos+1, 0);
            tmp.first = pos+2;
            if(hash.find(tmp) == hash.end()){
                hash.insert(tmp);
                q.push(tmp);
                step[++tail] = pos+1;
                pre[tail] = head;
                mid[tail] = pos;
                if(tmp.second==target && tmp.first==n)  return;
            }
        }
        if(pos >= 2){
            tmp.second = key;
            if(key&(1<<pos-1))  mkbit(tmp.second, pos-2, 1);
            else  mkbit(tmp.second, pos-2, 0);
            if(key&(1<<pos-2))  mkbit(tmp.second, pos-1, 1);
            else  mkbit(tmp.second, pos-1, 0);
            tmp.first = pos-2;
            if(hash.find(tmp) == hash.end()){
                hash.insert(tmp);
                q.push(tmp);
                step[++tail] = pos-2;
                pre[tail] = head;
                mid[tail] = pos;
                if(tmp.second==target && tmp.first==n)  return;
            }
        }
        head++;
    }
}

void Print(int tail){
    if(tail == 0)  return;
    else{
        Print(pre[tail]);
        ans.push_back(make_pair(mid[tail], step[tail]));
    }
}

int main()
{
    freopen("shuttle.in", "r", stdin);
    //freopen("shuttle.out", "w", stdout);
    scanf("%d", &n);
    //pre init
    start = target = 0;
    for(int i = 1; i <= n; i++)  start = (start<<1) + 1;
    target = (start<<n);
    q.push(make_pair(n, start));
    hash.insert(make_pair(n, start));
    b_bfs();
    Print(tail);
    pair <short, int> tmp;
    for(int i = 0; i < ans.size(); i++){
        tmp = ans[i];
        if(i && i%20==0)  printf("\n");
        if(i%20)  putchar(' ');
        printf("%d", 2*n-tmp.second+(tmp.first>tmp.second ? 1:0));
    }
    printf("\n");
    return 0;
}

找规律的AC代码：

/*
ID: zlqest11
LANG: C++
TASK: shuttle
*/

#include <iostream>
#include <cstdio>
#include <cstring>
using namespace std;

int n, ct, table[500], ans[500];

int main()
{
    freopen("shuttle.in", "r", stdin);
    freopen("shuttle.out", "w", stdout);
    scanf("%d", &n);
    if(n == 1)  printf("1 3 2\n");
    else{
        ct = 0;
        if(n & 1){
            for(int i = 3; i <= n; i += 2){
                table[++ct] = 1;
                for(int j = 1; j < i; j++)  table[++ct] = -2;
                table[++ct] = -1;
                for(int j = 1; j <= i; j++)  table[++ct] = 2;
            }
            for(int i = ct-n; i > 0; i--)
                table[++ct] = table[i];
        }
        else{
            for(int i = 2; i <= n; i += 2){
                table[++ct] = 1;
                for(int j = 1; j <= i; j++)  table[++ct] = -2;
                if(i == n)  break;
                table[++ct] = -1;
                for(int j = 1; j <= i+1; j++)  table[++ct] = 2;
            }
            for(int i = ct-n; i > 0; i--)  table[++ct] = table[i];
        }
        //for(int i = 1; i <= ct; i++)  printf("%d ", table[i]);
        //printf("\n");
        int tail = 0;
        ans[++tail] = n;
        ans[++tail] = ans[tail-1]+2;
        for(int i = 1; i <= ct; i++)
            ans[++tail] = ans[tail-1] + table[i];
        ans[++tail] = ans[tail-1] + 2;
        ans[++tail] = ans[tail-1] - 1;
        for(int i = 1; i <= tail; i++){
            if(i%20 != 1)  putchar(' ');
            printf("%d", ans[i]);
            if(i%20 == 0)  printf("\n");
        }
        if(tail%20 != 0)  printf("\n");
    }
    return 0;
}