Leetcode 118, Yanghui triangle

118、 Yang hui triangle

1） Title Description

Given a nonnegative integer *numRows,* Generate 「 Yang hui triangle 」 Before numRows That's ok .

stay 「 Yang hui triangle 」 in , Each number is the sum of the numbers at the top left and right of it .

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]]

Tips :

• 1 <= numRows <= 30

2） analysis

iteration . The elements at both ends of each line and the elements in the middle are calculated separately . At both ends of each line are 1, Iterative calculation of the elements in the middle , Use the previous line to calculate the next line .

• Yanghui triangle's No i Yes i Elements ;
• The first element and the last element of each line of Yang Hui triangle are 1;
• The third line of Yang Hui triangle begins , Elements not at both ends res[i][j]=res[i-1][j-1]+res[i-1][j];

3）C++ Code

class Solution {
    
public:
    vector<vector<int>> generate(int numRows) {
    
        vector<vector<int>> res(numRows);
        for(int i=0;i<numRows;i++){
    
            res[i].resize(i+1);
            res[i][0]=res[i][i]=1;
            for(int j=1;j<i;j++){
    
                res[i][j]=res[i-1][j-1]+res[i-1][j];
            }
        }
        return res;
    }
};