题面：https://www.luogu.com.cn/problem/P1825

这题显然是BFS，于是光速写完，交了一发WA了。

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#include <cctype>
#include <cstdio>
#include <cstdlib>
#include <iostream>
#include <ostream>
#include <queue>
#include <unordered_map>
#include <utility>
#include <vector>
using namespace std;
const int MAX_SIZE = 304;
int n, m;
char g[MAX_SIZE][MAX_SIZE];
int dir[4][2] = {{-1, 0}, {1, 0}, {0, -1}, {0, 1}};
int s_row, s_col, e_row, e_col;
unordered_map<char, vector<pair<int, int>>> trans;
bool vis[MAX_SIZE][MAX_SIZE];
struct State
{
    int row;
    int col;
    int step;
};
int main(int argc, const char *argv[])
{
    cin >> n >> m;
    for (int i = 1; i <= n; i++)
    {
        scanf("%s", g[i] + 1);
        for (int j = 1; j <= m; j++)
        {
            if (g[i][j] == '@')
            {
                s_row = i;
                s_col = j;
            }
            else if (g[i][j] == '=')
            {
                e_row = i;
                e_col = j;
            }
            else if (isalpha(g[i][j]))
            {
                trans[g[i][j]].emplace_back(make_pair(i, j));
            }
        }
    }
    queue<State> q;
    q.emplace(State{s_row, s_col, 0});
    vis[s_row][s_col] = true;
    while (!q.empty())
    {
        State old = q.front();
        q.pop();
        for (int d = 0; d < 4; ++d)
        {
            int n_row = old.row + dir[d][0];
            int n_col = old.col + dir[d][1];
            if (n_row < 1 || n_row > n || n_col < 1 || n_col > m)
            {
                continue;
            }
            else if (vis[n_row][n_col])
            {
                continue;
            }
            else if (g[n_row][n_col] == '#')
            {
                continue;
            }
            vis[n_row][n_col] = true;
            if (isalpha(g[n_row][n_col]))
            {
                auto poses = trans[g[n_row][n_col]];
                if (poses[0].first == n_row && poses[0].second == n_col)
                {
                    n_row = poses[1].first;
                    n_col = poses[1].second;
                }
                else
                {
                    n_row = poses[0].first;
                    n_col = poses[0].second;
                }
                vis[n_row][n_col] = true;
            }
            if (n_row == e_row && n_col == e_col)
            {
                cout << old.step + 1 << endl;
                exit(0);
            }
            q.emplace(State{n_row, n_col, old.step + 1});
        }
    }
    return 0;
}

看了题解后大彻大悟：

上面的代码标记了传送点的vis，这实际上是多此一举。比如下面这个例子，传送点A是作为中转点的，实际上走了两次。

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#	#	#	#	#	#	=	#	#
#	.	.	.	.	.	.	#	#
#	#	#	A	#	#	#	#	#
#	@	.	.	#	A	.	.	#
#	#	#	#	#	#	#	#	#