108. Convert Sorted Array to Binary Search Tree

题目

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

For this problem, a height-balanced binary tree is defined as a binary tree in which the depth of the two subtrees of every node never differ by more than 1.

Example

Given the sorted array: [-10,-3,0,5,9],

One possible answer is: [0,-3,9,-10,null,5], which represents the following height balanced BST:

      0
     / \
   -3   9
   /   /
 -10  5

算法

一开始想复杂了，以为要动态的去改变树的结构是它变成平衡二叉树，其实很简单，因为是排好序的数组，只需要每次取中值，然后递归左子树，递归右子树即可，这样构成的二叉树一定是平衡的。

代码

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     struct TreeNode *left;
 *     struct TreeNode *right;
 * };
 */
struct TreeNode* sortToBST(int* nums, int left, int right){
    if(left>right){
        return NULL;
    }
    int mid = (left + right) / 2;
    struct TreeNode* root = (struct TreeNode*)malloc(sizeof(struct TreeNode));
    root->val = nums[mid];
    root->left = sortToBST(nums, left, mid - 1);
    root-> right = sortToBST(nums, mid + 1, right);
    
    return root;
}

struct TreeNode* sortedArrayToBST(int* nums, int numsSize) {
    return sortToBST(nums, 0, numsSize-1);
}