04_帕斯卡三角形

# 用时0ms
class Solution(object):
    def generate(self, numRows):
        """
        :type numRows: int
        :rtype: List[List[int]]
        """
        # 第 n 行要用到 n-1 行的数据，应该是个动态规划吧
        # n = [1 n-1[0]+n-1[1] .... n-1[n-2]+n-1[n-1]]
        if numRows == 0:
            return []
        if numRows == 1:
            return [[1]]
        if numRows == 2:
            return [[1], [1, 1]]

        list1 = [[1], [1, 1]]
        for i in range(3, numRows + 1):
            list1.append(self.help(list1))

        return list1

    # 已知第n行 求n+1行
    def help(self, list1):
        new = []
        for j in range(len(list1[-1]) - 1):
            new.append(list1[-1][j] + list1[-1][j + 1])
        new = [1] + new + [1]
        return new


# leetcode 最优解 0ms
class Solution2(object):
    def generate(self, numRows):
        """
        :type numRows: int
        :rtype: List[List[int]]
        """
        if numRows == 0:
            return []
        if numRows == 1:
            return [[1]]
        res = [[1]]
        def recursion(row):

            if row == numRows:
                return
            res.append([])
            # 在开头添加一个1
            res[row].append(1)
            for i in range(row-1):
                tmp = res[row-1][i]+res[row-1][i+1]
                res[row].append(tmp)
            # 在最后添加一个1
            res[row].append(1)
            recursion(row+1)
        recursion(1)
        return res


s = Solution()
print(s.generate(5))