dfs wa了4，6，80分救助，悬赏关注一个

#include<iostream>
#include<cmath>
using namespace std;
int mp[110][110],u[110][110],n,m;
int xx[4] = { -1,1,0,0 };
int yy[4] = { 0,0,-1,1 };
void dfs(int dx, int dy)
{
    for (int h = 0; h < 4; h++)
    {
        int x = dx + xx[h], y = dy + yy[h];
        if(x>0&&x<=n&&y>0&&y<=m)
        if (mp[x][y] < mp[dx][dy])
        {
            if (u[x][y] == 0) {//?
                u[x][y] = 1;
                dfs(x, y);
                u[dx][dy] = max(u[x][y] + 1, u[dx][dy]);//回溯
            }
            else {
                u[dx][dy] = max(u[x][y] + 1, u[dx][dy]);
            }
        }
    }
}
int main()
{
    cin >> n >> m;
    for (int i = 1; i <= n; i++)
        for (int j = 1; j <= m; j++)
            cin >> mp[i][j];
    int sum = 0;
    for (int i = 1; i <= n; i++)
        for (int j = 1; j <= m; j++)
            if (u[i][j] == 0) {
                dfs(i, j);
            }
    for (int i = 1; i <= n; i++)
        for (int j = 1; j <= m; j++)
            sum = max(u[i][j], sum);
    cout << sum << endl;
    return 0;
}