118.md

118. 杨辉三角

输入: 5
输出:
[
     [1],
    [1,1],
   [1,2,1],
  [1,3,3,1],
 [1,4,6,4,1]
]

思路

m = n + 1
a2, a1
a2[0] = 0 + a[0]
a2[1] = a1[0] + a1[1]

a1[1,1]
a2[1] = a1[0] + a1[1]
a2[1,2,1]

代码

# Python3
class Solution:
    def generate(self, numRows: int) -> List[List[int]]:
        if numRows == 0:
            return []
        if numRows == 1:
            return [[1]]
        if numRows == 2:
            return [[1],[1,1]]
        a1 = [1,1]
        res = [[1],[1,1]]
        for i in range(2, numRows):
            a2 = [1]
            for n in range(1, len(a1)):
                # a2[n] = a1[n-1] + a1[n]
                a2.append(a1[n-1] + a1[n])
            # a2[len(a1)] = 1
            a2.append(1)
            res.append(a2)
            a1 = a2
        return res

// Java
class Solution {
    public List<List<Integer>> generate(int numRows) {
        if (numRows == 1) {
            List<Integer> list = new List<>{{add(1);}};
            return new List<List<Integer>>().add(list);
        }
        if (numRows == 2) {
            List<Integer> list = new List<>{{add(1);}};
            List<Integer> list2 = new List<>{{add(1);add(1);}};
            return new List<List<Integer>>().add(list).add(list2);
        }
        
    }
}