【二叉树】108. Convert Sorted Array to Binary Search Tree

 

/**
 * 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:
    TreeNode* sortedArrayToBST(vector<int>& nums) {
    	if(nums.size()==0)
    		return NULL;

    	return bst(nums,0,nums.size()-1);
        
    }
    TreeNode* bst(vector<int> &nums,int begin,int end)
    {
    	int rootNum = findRoot(nums,begin,end);
    	if(rootNum==-1)
    		return NULL;
    	
    	TreeNode* root=(TreeNode*)malloc(sizeof(TreeNode));
    	root->val=nums[rootNum];
    	root->left=bst(nums,begin,rootNum-1);
    	root->right=bst(nums,rootNum+1,end);
    	return root;
    }
    int findRoot(vector<int> &nums,int begin,int end)
    {
    	if(begin>end)
    		return -1;
    	if(begin==end)
    		return begin;
    	if(begin + 1 == end)
    	{
    		if(nums[begin]<nums[end])
    			return end;
    		else
    			return begin;
    	}

    	return (begin+end)/2;
    }
};