108. Convert Sorted Array to Binary Search Tree*

108. Convert Sorted Array to Binary Search Tree*

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

題目描述

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

C++ 實現 1

前向遍歷. 從數組中間取值, 可以保證構建出來的樹是平衡的.

/**
 * 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 {
private:
    TreeNode* sortedArrayToBST(vector<int> &nums, int start, int end) {
        if (start > end)
            return nullptr;

        int mid = start + (end - start) / 2;
        TreeNode *root = new TreeNode(nums[mid]);
        root->left = sortedArrayToBST(nums, start, mid - 1);
        root->right = sortedArrayToBST(nums, mid + 1, end);
        return root;
    }
public:
    TreeNode* sortedArrayToBST(vector<int>& nums) {
        return sortedArrayToBST(nums, 0, nums.size() - 1);
    }
};