func uniquePaths(m int, n int) int {
	dp := make([][]int, n)
	for i := 0; i < n; i++ {
		dp[i] = make([]int, m)
	}

	// 直接初始化第一行, 答案固定为 1
	for i := 0; i < m; i++ {
		dp[0][i] = 1
	}

	for line := 1; line < n; line++ {
		dp[line][0] = 1 // 最左边的确定为 1
		for idx := 1; idx < m; idx++ {
			dp[line][idx] = dp[line][idx-1] + dp[line-1][idx]
		}
	}

	//fmt.Println(dp)
	return dp[n-1][m-1]
}