哪里错了啊啊，一直WA……

#include <iostream>
#include <queue>
#include <string.h>
using namespace std;

struct Node{int x, y, num;};

queue<Node> q;
char tp[2001][2001];
int a[2001][2001], ans1[2001][2001], n, m;
char ch[4] = {'D', 'R', 'L', 'U'};
string ans2;
int wk[4][2] = {{0, 1}, {1, 0}, {-1, 0}, {0, -1}};

void print(int ux, int uy)
{
	for (int i = 0; i < 4; i++)
	{
		int x = ux + wk[i][0], y = uy + wk[i][1];
		if (ans1[x][y] == ans1[ux][uy] + 1 && a[x][y] != a[ux][uy] && x >= 0 && x < n && y >= 0 && y < m)
		{
			print(x, y);
			cout << ch[i];
			break;
		}
	}
}

void solve()
{
    memset(ans1, -1, sizeof(ans1));
    ans2 = "";
    cin >> n >> m;
    for (int i = 0; i < n; i++)
        for (int j = 0; j < m; j++)
        {
        	cin >> tp[i][j];
        	a[i][j] = tp[i][j] - '0';
		}
    q.push((Node){0, 0, a[0][0]});
    ans1[0][0] = 0;
    while (!q.empty())
    {
        Node u = q.front();
        int ux = u.x, uy = u.y;
        q.pop();
        for (int i = 0; i < 4; i++)
        {
            int x = ux + wk[i][0], y = uy + wk[i][1];
            if (x < 0 || y < 0 || x > n || y > m || a[x][y] == a[ux][uy] || ans1[x][y] != -1) continue;
            ans1[x][y] = ans1[ux][uy] + 1;
            q.push((Node){x, y});
        }
    }
    cout << ans1[n-1][m-1] << endl;
    if (ans1[n-1][m-1] != -1)
    {
    	print(0, 0);
        cout << endl;
	}
}

int main()
{
    int T;
    cin >> T;
    while (T--) solve();
}

怀疑是最后输出路线错了，调了半天还是错的……