LeetCode (13) Pascal's Triangle (杨辉三角 )

题目描述

Given numRows, generate the first numRows of Pascal’s triangle.

For example, given numRows = 5,
Return

从第三行开始，每行除了最左边和最右边两个数为1，其他数字都是上一行中相邻两个数字之和。根据上述规则可以写出下面的代码：

class Solution {public:    vector<vector<int> > generateRow1() {        vector<int> row;        row.push_back(1);        vector<vector<int>> r;        r.push_back(row);        return r;    }    vector<vector<int> > generateRow2() {        vector<vector<int>> r = generateRow1();        vector<int> row;        row.push_back(1);        row.push_back(1);        r.push_back(row);        return r;    }    vector<vector<int> > generate(int numRows) {        if (numRows < 1)        {            vector<vector<int> > r;            return r;        }        if (numRows == 1)            return generateRow1();        if (numRows == 2)            return generateRow2();        vector<vector<int> > r = generateRow2();        for (int i = 2; i != numRows; ++i)        {            vector<int> row;            row.push_back(1);            for(int j = 1; j != i; ++j)            {                int next = r[i-1][j-1] + r[i-1][j];                row.push_back(next);            }            row.push_back(1);            r.push_back(row);        }        return r;    }};