算法问题实战策略 SORTGAME

地址 https://algospot.com/judge/problem/read/SORTGAME

 

 解答 

常规BFS是会超时的  按照书上的提示 应该是打表(居然还有提倡打表的题目)

tle 代码

#include <iostream>
#include <vector>
#include <algorithm>
#include <queue>
#include <map>

using namespace std;

int n, m;


int bfs(const vector<int>& v)
{
    vector<int> sorted = v;
    sort(sorted.begin(), sorted.end());
    queue<vector<int>> q;
    map<vector<int>, int> distance;
    distance[v] = 0;
    q.push(v);
    while (!q.empty()) {
        vector<int> here = q.front();
        q.pop();

        if (here == sorted) return distance[here];
        int cost = distance[here];
        //翻转可能的子区间
        for (int i = 0; i < v.size(); i++) {
            for (int j = i + 2; j <= v.size(); ++j) {
                reverse(here.begin() + i, here.begin() + j);
                if (distance.count(here) == 0) {
                    distance[here] = cost + 1;
                    q.push(here);
                }
                reverse(here.begin() + i, here.begin() + j);
            }
        }
    }

    return -1;
}


int main()
{
    cin >> n;

    while (n--) {
        cin >> m;
        vector<int> v;
        for (int i = 0; i < m; i++) {
            int t;
            cin >> t;
            v.push_back(t);
        }

        cout << bfs(v) << endl;
    }


    return 0;
}

ac代码

#include <iostream>
#include <algorithm>
#include <map>
#include <queue>
#include <vector>

using namespace std;

map<vector<int>, int> toSort;

void precalc(int n) {
    vector<int> perm(n);
    for (int i = 0; i < n; ++i) perm[i] = i;
    queue<vector<int>> q;
    q.push(perm);
    toSort[perm] = 0;
    while (!q.empty()) {
        vector<int> here = q.front();
        q.pop();
        int cost = toSort[here];
        for (int i = 0; i < n; i++) {
            for (int j = i + 2; j <= n; ++j) {
                reverse(here.begin() + i, here.begin() + j);
                if (toSort.count(here) == 0) {
                    toSort[here] = cost + 1;
                    q.push(here);
                }
                reverse(here.begin() + i, here.begin() + j);
            }
        }
    }
}

int solve(const vector<int>& perm)
{
    int n = perm.size();
    vector<int> fixed(n);
    for (int i = 0; i < n; ++i) {
        int smaller = 0;
        for (int j = 0; j < n; ++j) 
            if (perm[j] < perm[i])
                ++smaller;
            fixed[i] = smaller;
    }
    return toSort[fixed];
}


int n, m;
int main()
{
    cin >> n;
    while (n--) {
        cin >> m;
        precalc(m);
        vector<int> perm;
        for (int i = 0; i < m; i++) {
            int t;
            cin >> t;
            perm.push_back(t);
        }
        cout << solve(perm) << endl;
    }


    return 0;
}