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<string>& 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<string> letterCombinations(string digits) {vector<string> v;if(digits.size() == 0){return {};}int di = 0;Combinations(digits,di,"",v);return v;}
};

杨辉三角 

class Solution {
public:vector<vector<int>> generate(int numRows) {vector<vector<int>> vv;vv.resize(numRows,vector<int>());//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;}
};