118.Pascal's Triangle

Posted on | In

Question

Given a non-negative integer numRows, generate the first numRows of Pascal’s triangle.

img
In Pascal’s triangle, each number is the sum of the two numbers directly above it.

Example:

1
2
3
4
5
6
7
8
9
Input: 5
Output:
[
     [1],
    [1,1],
   [1,2,1],
  [1,3,3,1],
 [1,4,6,4,1]
]

Answer

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
class Solution {
    public List<List<Integer>> generate(int numRows) {
        List<List<Integer>> res = new ArrayList<List<Integer>>();
        for(int i=0;i<numRows;i++){
            List<Integer> temp = new ArrayList<>();
            for(int j=0;j<i+1;j++){
                if(j==0 || j==i)
                    temp.add(1);
                else
                    temp.add(res.get(i-1).get(j-1)+res.get(i-1).get(j));
            }
            res.add(temp);
        }
        return res;
    }
}

Running time：O(n^2)

Space：O(n^2)