leetcode Convert Sorted Array to Binary Search Tree

Convert Sorted Array to Binary Search Tree

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

本题是二分法的高级应用，每次选择root都是从数列的中间选择，那么最后构造出来的二叉树一定是高度平衡的BST了。

/**
 * Definition for binary tree
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution 
{
public:
	TreeNode *sortedArrayToBST(vector<int> &num) 
	{
		return biSortedArrayToBST(num, 0, num.size()-1);
	}

	TreeNode *biSortedArrayToBST(vector<int> &num, int low, int up)
	{
		if (low > up) return NULL;
		int mid = low + ((up-low)>>1);
		TreeNode *r = new TreeNode(num[mid]);

		r->left = biSortedArrayToBST(num, low, mid-1);
		r->right = biSortedArrayToBST(num, mid+1, up);

		return r;
	}
};

//2014-2-16 update
	TreeNode *sortedArrayToBST(vector<int> &num) 
	{
		return toBST(num, 0, num.size()-1);//下标num.size()-1
	}
	TreeNode *toBST(vector<int> &num, int n1, int n2)
	{
		if (n1 > n2) return nullptr;
		int mid = n1 + ((n2-n1)>>1);
		TreeNode *root = new TreeNode(num[mid]);

		root->left = toBST(num, n1, mid-1);
		root->right = toBST(num, mid+1, n2);
		return root;
	}