leetcode -- Convert Sorted Array to Binary Search Tree

Given an array where elements are sorted in ascending order, convert it to a height balanced BST.

[解题思路]

递归确定每棵树的根节点的值，这里根节点的值是二分搜索的中值

由于这里是有序数组，确定root的时间复杂度为O(1), 整个算法的时间复杂度为O(n)，n为节点数目。

/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public TreeNode sortedArrayToBST(int[] num) {
        // Start typing your Java solution below
        // DO NOT write main() function
        return generate(num, 0, num.length - 1);    
    }
    
    public TreeNode generate(int[] num, int start, int end){
        if(start > end){
            return null;
        }
        int mid = (end + start) / 2;
        TreeNode root = new TreeNode(num[mid]);
        root.left = generate(num, start, mid - 1);
        root.right = generate(num, mid + 1, end);
        return root;
    }
}