03. pascals's triangle.java

/*
Given an integer numRows, return the first numRows of Pascal's triangle.

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


 

Example 1:

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

Input: numRows = 1
Output: [[1]]
 

Constraints:

1 <= numRows <= 30
*/
class Solution {
    public List<List<Integer>> generate(int numRows) {
        List<List<Integer>> res= new ArrayList<List<Integer>>();
        List<Integer>row, pre=null;
        
        for(int i=0;i<numRows;++i){
        
            row =  new ArrayList<Integer>();
            for(int j=0;j<=i;++j)
                if(j==0 || j==i)
                row.add(1);
                else
                   row.add(pre.get(j-1)+pre.get(j));
             pre= row;
            res.add(row);
            }
        return res;
        
        }
    }