108. Convert Sorted Array to Binary Search Tree

Level : Easy

原題連結 : Click

題目 :

Given an integer array nums where the elements are sorted in ascending order, convert it to a height-balanced binary search tree.

A height-balanced binary tree is a binary tree in which the depth of the two subtrees of every node never differs by more than one.

Example :

解題思路 :

• 這題需要我們將遞增序列的數列轉換成height-balacend的BST，題目定義為每一個node所連接的左右subtree之間深度不能差超過1
• 其實呢這題考察我們對BST的熟悉度跟Recursive的熟練度
• 若要左右高度相仿的subtree，那該root必須是該遞增序列的中位數才能使得左右subtree node數平均分布，然後每一個node都遵守與root一樣的準則寫出recursive code

Recursive解法

• time complexity: O(n) , space complexity: O(1)

Runtime: 0 ms, faster than 100.00% of Go online submissions for Convert Sorted Array to Binary Search Tree.

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/**
 * Definition for a binary tree node.
 * type TreeNode struct {
 *     Val int
 *     Left *TreeNode
 *     Right *TreeNode
 * }
 */
func sortedArrayToBST(nums []int) *TreeNode {
    
    if len(nums) == 0{
        return nil
    }
    
    left := 0
    right := len(nums)-1
    mid := (right + left)/2
    
    node := &TreeNode{
        Val: nums[mid],
        Left: sortedArrayToBST(nums[left:mid]),
        Right: sortedArrayToBST(nums[mid+1:right+1]),
    }
    
    return node
}

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