leetcode 62不同路径

首先建立一个m*n的数组，数组中的值是能到达此格子的路线数，题目规定只能向下或向右移动，所以第一行和第一列的格子只能从其左侧或上侧移动而来，所以第一行和第一列的值为1，其余格子值为其左侧和上侧格子值之和

class Solution:
    def uniquePaths(self, m: int, n: int) -> int:
        if m == 0 or n == 0:
            return None
        if m == 1 and n == 1:
            return 1
        tem = [[0 for i in range(n)] for j in range(m)]
        for i in range(m):
            for j in range(n):
                if i == 0 or j == 0:
                    tem[i][j] = 1
                else:
                    tem[i][j] = tem[i-1][j] + tem[i][j-1]
        return tem[m-1][n-1]