Leetcode(108) Convert Sorted Array to Binary Search Tree

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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:

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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

解法

左右递归分别建树即可。注意终止条件的处理

具体代码如下：

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class Solution {
    public TreeNode sortedArrayToBST(int[] nums) {
        List l = new ArrayList<Integer>();
        for (int i :
                nums) {
            l.add(i);
        }
        return helper(l, 0, l.size() - 1);
    }

    TreeNode helper(List nums, int start, int end) {
        if (start > end) {
            return null;
        }
        int val = (int) nums.get((start + end) / 2);
        TreeNode root = new TreeNode(val);
        root.left = helper(nums, start, (start + end) / 2 - 1);
        root.right = helper(nums, (start + end) / 2 + 1, end);
        return root;
    }
}