Leetcode-Pascal's 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]
]


Solution:

public class Solution {
    public List<List<Integer>> generate(int numRows) {
        List<List<Integer>> tri = new ArrayList<List<Integer>>();
        List<Integer> curRow = new ArrayList<Integer>();
        if (numRows==0)
            return tri;
        
        curRow.add(1);
        tri.add(curRow);
        int len;
        for (int i=1;i<numRows;i++){
            List<Integer> preRow = tri.get(i-1);
            curRow = new ArrayList<Integer>();
            len = i+1;
            //j==0
            curRow.add(1);
            //j==1 to (len-2)
            for (int j=1;j<len-1;j++)
                curRow.add(preRow.get(j-1)+preRow.get(j));
            //j==len-1
            curRow.add(1);
            tri.add(curRow);
        }
        
        return tri;
    }
}

这是一个简单的递推问题。