LeetCode 108. Convert Sorted Array to Binary Search Tree（二叉樹）

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

問題描述

108. Convert Sorted Array to Binary Search Tree

Easy

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:

     / \

   -3   9

   /   /

 -10  5

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題意

根據有序數組建立二叉搜尋樹，要求二叉樹的近似平衡性，即各個葉子節點的深度至多相差1.

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思路

遞歸

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代碼

/**
 * 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) {
        return build(nums, 0, nums.size());
    }
    
    /**
    * build subtree from nums[left:right]
    * return:
    *   root node pointer of the subtree built
    */
    TreeNode* build(vector<int>& nums, int left, int right) {
        if (left == right) {
            return NULL;
        }
        int mid = (left+right)/2;
        TreeNode* root = new TreeNode(nums[mid]);
        root->left = build(nums, left, mid);
        root->right = build(nums, mid+1, right);
        return root;
    }
};