[LeetCode] Pascal' Triangle

Given numRows, generate the first numRows of Pascal's triangle.
For example, given numRows = 5,
Return

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

Java Code

public List> generate(int numRows) {
        List> res = new ArrayList>();       
        List first = new ArrayList();
        if(numRows < 1) return res;
        
        first.add(1);
        res.add(first);
        
        for(int i = 1; i < numRows; i++){
            List prev = new ArrayList();
            prev = res.get(i - 1);
            
            List curr = new ArrayList();
            
            for(int j = 0; j <= prev.size(); j++){
                int num = 0;
                
                if(j < prev.size()) num += prev.get(j);
                if(j > 0) num += prev.get(j-1);
                
                curr.add(num);
            }
            
            res.add(curr);
        }
        
        return res;
    }