Leetcode 118. 杨辉三角（动态规划）

描述

给定一个非负整数 numRows，生成杨辉三角的前 numRows 行。

在杨辉三角中，每个数是它左上方和右上方的数的和。

示例:

输入: 5
输出:

[
     [1],
    [1,1],
   [1,2,1],
  [1,3,3,1],
 [1,4,6,4,1]
]

Solution

动态规划递推。

class Solution:
    def generate(self, num_rows):
        triangle = []

        for row_num in range(num_rows):
            row = [None for _ in range(row_num+1)]
            row[0], row[-1] = 1, 1

            # Each triangle element is equal to the sum of the elements
            # above-and-to-the-left and above-and-to-the-right.
            for j in range(1, len(row)-1):
                row[j] = triangle[row_num-1][j-1] + triangle[row_num-1][j]

            triangle.append(row)

        return triangle