Main idea of the title:

Given a non-negative integer numRows, the first numRows row of Yang Hui triangle is generated.

In Yang Hui triangle, each number is the sum of its upper left and upper right.

Example:

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

Train of thought:

• The first step is to initialize the corresponding two-dimensional array according to the given numRows
• In the second part, according to the rules of Yang Hui triangle, the corresponding number is filled. For each inner list, the element of res[i][j] is equal to the value of res[i-1][j-1] plus the value of res[i-1][j].

class Solution(object):
    def generate(self, numRows):
        """
        :type numRows: int
        :rtype: List[List[int]]
        """
        res=[]
        for i in range(numRows):#One row per cycle
            temp=[1]*(i+1)#Initialized to 1
            res.append(temp)
            for j in range(1,i):#column
                res[i][j]=res[i-1][j-1]+res[i-1][j]
        return res

The following is the Java version:

1.	public static List<List<Integer>> generate(int numRows){  
2.	        List<List<Integer>> s = new ArrayList<List<Integer>>();  
3.	        if(numRows == 0) return s;  
4.	        ArrayList<Integer> a = new ArrayList<Integer>();  
5.	        a.add(1);     
6.	        s.add(a);   //Note that the two-dimensional assignment problem cannot be directly added 1  
7.	        for(int i = 1; i < numRows; i++){  
8.	            List<Integer> t = new ArrayList<Integer>();  
9.	            t.add(1);  
10.	            for(int j = 1; j <= i - 1 ; j++){  
11.	                t.add(s.get(i - 1).get(j - 1) + s.get(i - 1).get(j));  
12.	            }  
13.	            t.add(1);  
14.	            s.add(t);  
15.	        }  
16.	        return s;  
17.	    }  

Test:

1.	public static void main(String[] args){  
2.	      
3.	        System.out.println(generate(5));  
4.	    }