leetcode | 杨辉三角 | 电话号码配对

   电话号码的字母组合

class Solution {
    string _num[10] = {"","","abc","def","ghi","jkl","mno","pqrs","tuv","wxyz"};

public:
    void Combinations(const string& digits,int di,string numcom,vector& v)
    {
        if(di == digits.size())//结束条件
        {
            v.push_back(numcom);
            return ;
        }
        int num = digits[di] - '0';
        string str = _num[num];
        for(auto ch : str)
        {
            Combinations(digits,di+1,numcom+ch,v);//递归一定要注意numcom是+
            //不是+=；
        }
    }
    vector letterCombinations(string digits) {
        vector v;
        if(digits.size() == 0)
        {
            return {};
        }
        int di = 0;
        Combinations(digits,di,"",v);
        return v;
    }
};

杨辉三角 

class Solution {
public:
    vector> generate(int numRows) {
        vector> vv;
        vv.resize(numRows,vector());//进行初始化
        //进行的是每行初始化，因为这里表示的是顺序表里面是个顺序表
        for(int i = 0; i < vv.size(); i++)//初始化没列
        {
            vv[i].resize(i+1,0);
            vv[i][0] = vv[i][vv[i].size() - 1] = 1;
        }
        for(int i = 0 ;i < vv.size(); i++)
        {
            for(int j = 0; j < vv[i].size(); j++)
            {
                if(vv[i][j] == 0)
                {
                    vv[i][j] = vv[i-1][j-1] + vv[i-1][j];
                }
            }
        }
        return vv;
    }
};