118. Pascal's Triangle

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

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

Example:

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

按照规律brute force

时间：O(N^2)，空间：O(N^2)　　-- N : numRows

class Solution {
    public List<List<Integer>> generate(int numRows) {
        List<List<Integer>> res = new ArrayList<>();
        if(numRows == 0) return res;
        
        res.add(new ArrayList<>());
        res.get(0).add(1);
        for(int i = 1; i < numRows; i++) {
            List<Integer> tmp = new ArrayList<>();
            tmp.add(1);
            for(int j = 0; j + 1 < i; j++) {
                tmp.add(res.get(i - 1).get(j) + res.get(i - 1).get(j + 1));
            }
            tmp.add(1);
            res.add(tmp);
        }
        return res;
    }
}