优化时间和空间复杂度的优秀代码！

时间复杂度：O(H×W)
空间复杂度：O(W)

#include<bits/stdc++.h>
using namespace std;

const int MOD = 1000000007;

int main() {
    int H, W;
    cin >> H >> W;
    vector<string> grid(H);
    for (int i = 0; i < H; i++) {
        cin >> grid[i];
    }
    if (grid[0][0] == '#' || grid[H-1][W-1] == '#') {
        cout << 0 << endl;
        return 0;
    }
    vector<int> dp(W, 0);
    dp[0] = 1;  
    for (int i = 0; i < H; i++) {
        for (int j = 0; j < W; j++) {
            if (grid[i][j] == '#') {
                dp[j] = 0;  
                continue;
            }
            
            if (j > 0 && grid[i][j-1] != '#') {
                dp[j] = (dp[j] + dp[j-1]) % MOD;
            }
        }
    }
    cout << dp[W-1] << endl;
    return 0;
}

代码会超时和内存不够