[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]
]

利用帕斯卡三角的性质：除每行最左侧与最右侧的数字以外，每个数字等于它的左上方与右上方两个数字之和。

class Solution {
public:
    vector<vector<int>> generate(int numRows) {
        vector<vector<int>> res;
        if (numRows <= 0)
            return res;
        res.assign(numRows, vector<int>(1));
        for (int i = 0; i != numRows; i++) {
            res[i][0] = 1;
            if (i == 0)
                continue;
            for (int j = 1; j != i; j++) {
                res[i].push_back(res[i - 1][j - 1] + res[i - 1][j]);
            }
            res[i].push_back(1);
        }
        return res;
    }
};
// 3 ms

另一种简洁写法

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;
    }
};
// 3 ms

  相关题目： [LeetCode] Pascal's Triangle II