Leetcode_convert-sorted-array-to-binary-search-tree

地址：http://oj.leetcode.com/problems/convert-sorted-array-to-binary-search-tree/

Given an array where elements are sorted in ascending order, convert it to a height balanced BST.

思路：先找到头节点，再递归构造左子树和右子树。124ms。

参考代码：

/**
* Definition for binary tree
* struct TreeNode {
*     int val;
*     TreeNode *left;
*     TreeNode *right;
*     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
* };
*/
class Solution {
public:
void construct_tree(TreeNode**p, vector<int>vec)
{
if(vec.empty())
return;
int mid = vec.size()/2;
*p = new TreeNode(vec[mid]);
vector<int>left_vec, right_vec;
for(int i = 0; i<mid; ++i)
left_vec.push_back(vec[i]);
for(int i=mid+1; i<vec.size(); ++i)
right_vec.push_back(vec[i]);
construct_tree(&((*p)->left), left_vec);
construct_tree(&((*p)->right), right_vec);
}
TreeNode *sortedArrayToBST(vector<int> &num) {
if(num.empty())
return NULL;
TreeNode* head = NULL;
construct_tree(&head, num);
return head;
}
};

SECOND TRIAL, 60MS

class Solution {
private:
TreeNode* construct_tree(TreeNode* root, vector<int>& num, int begin, int end)
{
if(begin > end)
return NULL;
int mid = (begin+end)/2;
root = new TreeNode(num[mid]);
root->left = construct_tree(root->left, num, begin, mid-1);
root->right = construct_tree(root->right, num, mid+1, end);
return root;
}
public:
TreeNode *sortedArrayToBST(vector<int> &num) {
return construct_tree(NULL, num, 0, num.size()-1);
}
};