分治算法

class Solution {
public:
    double myPow(double x, int n) {
        if(n==0) return 1;
        if(n == -(1ll << 31)) return 1.0/(myPow(x,-(n+1))*x);
        if(n < 0) return 1.0/myPow(x,-n);
        double temp = myPow(x,n/2);
        double ans = temp*temp;
        if(n%2==1) ans *=x;
        return ans;
    }
};

class Solution {
public:
    double myPow(double x, long long n) {
        if(n==0) return 1;
        //if(n==-(1ll<<31)) return 1.0/myPow(x,-(n+1)*x);
        if(n<0) return 1.0/myPow(x,-n);
        double temp=myPow(x,n/2);
        double ans = temp*temp;
        if(n%2 == 1) ans*=x;
        return ans;
    }
};

class Solution {
public:
    vector<string> generateParenthesis(int n) {
        if(n == 0) return {""};
        if(store.find(n) != store.end()) return store[n];
        vector<string> ans;
        for(int k=1;k<=n;k++)
        {
            vector<string> A = generateParenthesis(k-1);
            vector<string> B = generateParenthesis(n-k);
            for(string & a:A)
            {
                for(string & b:B)
                {
                    ans.push_back("(" + a + ")" +b);
                }
            }
        }
        store[n]=ans;
        return ans;
    }
private:
    unordered_map<int ,vector<string>> store;
};