62.不同路径

62.不同路径
-< 往下n次往右m次，从起点开始需移动n-1和m-1次
    因此在n+m-2中选取n-1次，其余向右走
demo1:
class Solution:
    def uniquePaths(self, m: int, n: int) -> int:
        return comb(n+m-2,n-1)
-<动态规划
    f(i,j)=f(i-1,j)+f(i,j-1)
    每一个点来源于上面或者左边的点
demo2；
class Solution:
    def uniquePaths(self, m: int, n: int) -> int:
        dp = [[0]*n for _ in range(m)] #m行n列初始化
        if m==1 or n==1:
            return 1
        for j in range(1,n):
            dp[0][j] = 1 #边界复制
        for i in range(1,m):
            dp[i][0]=1
        for i in range(1,m):
            for j in range(1,n):
                dp[i][j] = dp[i-1][j]+dp[i][j-1]
        return  dp[m-1][n-1]