Unique Paths

class Solution(object):
    def uniquePaths(self, m, n):
        """
        :type m: int
        :type n: int
        :rtype: int
        """
        mat = [[1]*n]*m
        for i in range(m):
            for j in range(n):
                if i > 0 and j > 0:
                    mat[i][j] = mat[i-1][j] + mat[i][j-1]

        return mat[i][j]