leetcode 中等题(2)

50. Pow(x, n) (中等)

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

96. Unique Binary Search Trees(很快)

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

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 =  << n;
         vector<int> res(len,);
         for(int i = ;i != len;++i){
             res[i] =i ^ (i >> );
         }
         return res;
     }
 };