为啥我用pair存坐标，数组记录步数只有6分，案例都过不了，求助

#include<iostream>
#include<queue>
using namespace std;
int step[400][400];
queue<pair<int,int>>c;
char a[400][400];
bool vis[400][400];
int n, m;
int d[4][2]={{1,0},{-1,0},{0,1},{0,-1}};
int sx,sy;

void change(int &x, int &y) {
    for (int i = 1; i <= n; ++i) {
        for (int j = 1; j <= m; ++j) {
            if (a[i][j] == a[x][y] && (i != x || j != y)) {
                x = i;
                y = j;
                return;
            }
        }
    }
}

int main() {
    cin >> n >> m;
    for (int i = 1; i <= n; ++i) {
        for (int j = 1; j <= m; ++j) {
            cin >> a[i][j]; 
            if (a[i][j] == '@') {
                sx = i;
                sy = j;
            }
        }
    }

    c.push({sx, sy});
    vis[sx][sy] = true;
    step[sx][sy]=0;
    while (!c.empty()) {
      int dx=c.front().first;
      int dy=c.front().second;
        c.pop();

        if (a[dx][dy] == '=') {
            cout << step[dx][dy] << endl;
            return 0;
        }

        if (a[dx][dy] >= 'A' && a[dx][dy] <= 'Z') {
            change(dx, dy);
        }

        for (int i = 0; i < 4; ++i) {
            int nx = dx + d[i][0];
            int ny = dy + d[i][1];

            if (nx >= 1 && nx <= n && ny >= 1 && ny <= m && a[nx][ny] != '#' && !vis[nx][ny]) {
                vis[nx][ny] = true;
                c.push({nx,ny});
                step[nx][ny]=step[dx][dy]+1;
          
            }
        }
    }


    return 0;
}