杨辉三角 -- 迭代和递归分别实现

118. 杨辉三角

class YanghuiTriangle:
    """
    118. 杨辉三角
    https://leetcode.cn/problems/pascals-triangle/description/
    """
    def generate(self, numRows: int) -> List[List[int]]:
        """
        递归
        :param numRows:
        :return:
        """
        if numRows == 1:
            triangle = []
            firstRow = [1]
            triangle.append(firstRow)
            return triangle

        triangle = self.generate(numRows - 1)

        bottomRow = triangle[-1]
        newRow = [1]
        for i in range(len(bottomRow)-1):
            newRow.append(bottomRow[i] + bottomRow[i+1])

        newRow.append(1)
        triangle.append(newRow)

        return triangle

    def generate2(self, numRows: int) -> List[List[int]]:
        """
        迭代
        :param numRows:
        :return:
        """
        res = []
        if numRows < 1:
            return res
        # 先把第一层装进去作为 base case
        firstRow = [1]
        res.append(firstRow)
        # 开始一层一层生成，装入 res
        for i in range(2, numRows + 1):
            preRow = res[-1]
            res.append(self.generateNextRow(preRow))

        return res

        # 输入上一层的元素，生成并返回下一层的元素
    def generateNextRow(self, prevRow: List[int]) -> List[int]:
        curRow = [1]
        for i in range(1, len(prevRow)-1):
            curRow.append(prevRow[i] + prevRow[i+1])
        curRow.append(1)

        return curRow