leetcode 中等题(2)

50. Pow(x, n) (中等)

    double myPow(double x, int n) {
        double ans = 1;
        unsigned long long p;
        if (n < 0) {
            p = -n;
            x = 1 / x;
        } else {
            p = n;
        }
        while (p) {
            if (p & 1)
                ans *= x;
            x *= x;
            p >>= 1;
        }
        return ans;
    }

 96. Unique Binary Search Trees(很快)

class Solution {
public:
    int numTrees(int n) {
        long ans=1;
        for(int i=n+1;i<=2*n;i++)
            ans = i*ans/(i-n);
        return ans/(n+1);
    }
};

 94. Binary Tree Inorder Traversal(很快)

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<int> inorderTraversal(TreeNode* root) {
        stack<TreeNode*> s;
        vector<int> res;
        if(root==NULL) return res;
        TreeNode* p=root;
        while(!s.empty()||p){
            if(p){
                s.push(p);
                p=p->left;
            }
            else{
                p=s.top();
                s.pop();
                res.push_back(p->val);
                p=p->right;
            }
        }
        return res;
    }
};

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    vector<int> res;
    vector<int> inorderTraversal(TreeNode* root) {
        if(!root) return  res;
        inorderTraversal(root->left);
        res.push_back(root->val);
        inorderTraversal(root->right);
        return res;
    }
};

 89. Gray Code(很快)

class Solution {
public:
    vector<int> grayCode(int n) {
        int  len = 1 << n;
        vector<int> res(len,0);
        for(int i = 0;i != len;++i){
            res[i] =i ^ (i >> 1); 
        }
        return res;
    }
};