108.Convert Sorted Array to Binary Search Tree

Tags: Easy Tree

Links: https://leetcode.com/problems/convert-sorted-array-to-binary-search-tree/

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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

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/**
 * 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) {
        std::ios_base::sync_with_stdio(false);
        cin.tie(NULL);
        cout.tie(NULL);

        return build(nums, 0, (int)nums.size() - 1);
    }

    TreeNode *build(vector<int> & nums, int start, int end)
    {
        if (start > end) return NULL;

        int mid = start + ((end - start) >> 1);
        TreeNode *root = new TreeNode(nums[mid]);
        root -> left = build(nums, start, mid - 1);
        root -> right = build(nums, mid + 1, end);

        return root;
    }
};

一开始以为要手写AVL Tree，后来发现数组有序，那么一开始就可以找到根节点，然后递归实现。