DataStructure_Algorithm_HandBook_PreForLeetCode

108. Convert Sorted Array to Binary Search Tree

Given an integer array nums where the elements are sorted in ascending order, convert it to a

*height-balanced*

binary search tree.

Example 1:

Input: nums = [-10,-3,0,5,9]
Output: [0,-3,9,-10,null,5]
Explanation: [0,-10,5,null,-3,null,9] is also accepted:

Example 2:

Input: nums = [1,3]
Output: [3,1]
Explanation: [1,null,3] and [3,1] are both height-balanced BSTs.

Constraints:

• 1 <= nums.length <= 104
• -104 <= nums[i] <= 104
• nums is sorted in a strictly increasing order.

solution

首先题目要求高度平衡，所谓的高度平衡，就是左右节点的高度差不超过 1，这道题目也是一个建树的问题。

那最简单的方法，我们发现给的是有序数组，因此我们为了平衡，就取最中间的数字作为根节点 如果是偶数数组，就是相差1，如果是奇数数组，就是刚好相等。

var sortedArrayToBST = function(nums) {
   return buildTree(0,nums.length-1,nums);
};

function buildTree(l,r,nums){
    if(l > r) return null;
    let mid = (l+r) >> 1;
    let node = new TreeNode(nums[mid]);
    node.left = buildTree(l,mid-1,nums);
    node.right = buildTree(mid+1,r,nums);
    return node;
}