int uniquePaths1(int m, int n) {

if (m == 0 || n == 0) return 0;

vector<vector<int>> dp(m, vector<int>(n, 0));

for (int i = 0; i < m; i++) dp[i][0] = 1;

for (int j = 0; j < n; j++) dp[0][j] = 1;

for (int i = 1; i < m; i++) {

for (int j = 1; j < n; j++) {

dp[i][j] = dp[i - 1][j] + dp[i][j - 1];

}

}

return dp[m - 1][n - 1];

}

int uniquePaths(int m, int n) {

// 空间压缩

int dp[n];

for (int i = 0; i < m; i++) {

for (int j = 0; j < n; j++) {

if (i == 0 || j == 0) dp[j] = 1;

else dp[j] = dp[j-1] + dp[j];

}

}

return dp[n - 1];

}