[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.

 

分析： 我们知道，BST可以通过中序遍历生成一个sorted array，所以，我们能够通每次取中位数的方式生成root节点，将原始的这颗二叉树还原。同时，确定了中位数（root节点），左右字串就能够作为左右子树传递给递归传递参数。

思路： 二分递归转换

 

/**
* 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 *createTree(vector<int> &nums, int left, int right)
  {
    if(left > right)
      return NULL;

    int mid = (left + right)/2;
    TreeNode *root = new TreeNode(nums[mid]);
    root->left = createTree(nums, left, mid-1);
    root->right = createTree(nums, mid+1, right);
    
    return root;
  }

  TreeNode *sortedArrayToBST(vector<int> &nums)
  {
    return createTree(nums, 0, nums.size()-1);
  }
};