题目：

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]]

思路：

递归

package recursion;import java.util.List;import java.util.ArrayList;public class PascalsTriangle {    public List
     
      
       > generate(int numRows) {        List
       
        
         > res = new ArrayList
         
          
           >();; List
           
             row = new ArrayList
            
             (); if (numRows == 1) { row.add(1); res.add(row); } else if (numRows > 1){ res = generate(numRows - 1); List
             
               lastRow = res.get(numRows - 2); int size = lastRow.size(); for (int i = 0; i < size; ++i) { row.add((i > 0 ? lastRow.get(i - 1) : 0) + lastRow.get(i)); } row.add(lastRow.get(size - 1)); res.add(row); } return res; } public static void main(String[] args) { PascalsTriangle p = new PascalsTriangle(); List
              
               
                > res = p.generate(5); for (List
                
                  row : res) { for (int i : row) System.out.print(i + " "); System.out.println(); } }}