[Leetcode 108]有序数组转二叉搜索树Convert Sorted Array to Binary Search Tree

【题目】

给出的升序排序的数组，个数必为奇数，要求形成二叉搜索（平衡）树。

【思路】

辅助函数fun，[0，len]=>[0,mid-1]+[mid+1,len]。

当left>right,返回null。

    public TreeNode fun(int[] nums,int left,int right) {
        int mid=(right+left)/2;
        TreeNode tmp=new TreeNode(nums[mid]);
        if(right<left)
            return null;
        tmp.left=fun(nums,left,mid-1);
        tmp.right=fun(nums,mid+1,right);
        return tmp;
    }

【AC代码】

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public TreeNode sortedArrayToBST(int[] nums) {
        int len=nums.length;
        if(nums.length==0){
            return null;
        }
        TreeNode root=fun(nums,0,len-1);
        return root;
    }

    public TreeNode fun(int[] nums,int left,int right) {
        int mid=(right+left)/2;
        TreeNode tmp=new TreeNode(nums[mid]);
        if(right<left)
            return null;
        tmp.left=fun(nums,left,mid-1);
        tmp.right=fun(nums,mid+1,right);
        return tmp;
    }
}