Convert Sorted Array to Binary Search Tree

Convert Sorted Array to Binary Search Tree

问题：

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

思路：

　　递归，dfs

我的代码：

/**

 * 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) {

        if(num == null || num.length == 0 )

            return null ;

        int len = num.length ;

        TreeNode rst = arrayToBST(num, 0, len - 1);

        return rst;

    }

    public TreeNode arrayToBST(int [] num, int left, int right)

    {

        if(left > right)    return null;

        if(left == right)   return new TreeNode(num[left]);

        int mid = (left + right)/2;

        TreeNode root = new TreeNode(num[mid]);

        root.left = arrayToBST(num, left, mid - 1);

        root.right = arrayToBST(num, mid + 1 , right);

        return root;

    }

}

View Code

他人代码：

public TreeNode sortedArrayToBST(int[] num) {

    if (num.length == 0) {

        return null;

    }

    TreeNode head = helper(num, 0, num.length - 1);

    return head;

}



public TreeNode helper(int[] num, int low, int high) {

    if (low > high) { // Done

        return null;

    }

    int mid = (low + high) / 2;

    TreeNode node = new TreeNode(num[mid]);

    node.left = helper(num, low, mid - 1);

    node.right = helper(num, mid + 1, high);

    return node;

}

View Code

学习之处：

• 我的代码有冗余的地方 left == right的判断可以省去