[LeetCode] Convert Sorted Array to Binary Search Tree

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

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Tree Depth-first Search

方法一：递归，也是dfs

/**
* 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)
{
int size = num.size();
if(size == 0)
return NULL;
return sortedArrayToBSTInternal(num, 0, size - 1);

}

TreeNode *sortedArrayToBSTInternal(vector<int> &num, int low, int high)
{

// the code is very important, i.e: low = 4, hight = 5, mid = 4,
// will call sortedArrayToBSTInternal(num, 4, 3)
if(low > high)
return NULL;
if(low == high)
return new TreeNode(num[low]);

int mid = (high-low)/2 + low;

TreeNode *root = new TreeNode(num[mid]);
TreeNode *left = sortedArrayToBSTInternal(num, low, mid - 1);
TreeNode *right = sortedArrayToBSTInternal(num, mid + 1, high);
root->left = left;
root->right= right;
return root;
}
};