[LeetCode] 108. Convert Sorted Array to Binary Search Tree

题目链接：传送门

Description

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<br>
 / \<br>

-3   9


/   /


-10  5

Solution

题意：

给定一个有序序列，将其转化成一棵平衡二叉树

思路：

观察一下可以发现，每次将序列拆为两半去构造是合理的

/**
 * 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* solve(vector<int>& nums, int l, int r) {
        if (l > r)  return NULL;
        if (l == r)  return new TreeNode(nums[l]);
        int mid = l + r >> 1;
        TreeNode* cur = new TreeNode(nums[mid]);
        cur->left = solve(nums, l, mid - 1);
        cur->right = solve(nums, mid + 1, r);
        return cur;
    }
    TreeNode* sortedArrayToBST(vector<int>& nums) {
        return solve(nums, 0, nums.size() - 1);
    }
};