题解 | #将升序数组转化为平衡二叉搜索树#

import java.util.*;

/*
 * public class TreeNode {
 *   int val = 0;
 *   TreeNode left = null;
 *   TreeNode right = null;
 *   public TreeNode(int val) {
 *     this.val = val;
 *   }
 * }
 */

/**
 * NC11 将升序数组转化为平衡二叉搜索树
 * @author d3y1
 */
public class Solution {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     * 程序入口
     *
     * @param nums int整型一维数组
     * @return TreeNode类
     */
    public TreeNode sortedArrayToBST (int[] nums) {
        return solution(nums);
    }

    /**
     * dfs
     * @param nums
     * @return
     */
    private TreeNode solution(int[] nums){
        return arrayToBST(nums, 0, nums.length-1);
    }

    /**
     * 递归
     * @param nums
     * @param left
     * @param right
     * @return
     */
    private TreeNode arrayToBST(int[] nums, int left, int right){
        if(left > right){
            return null;
        }
        int mid = (left+right)/2;
//        int mid = (left+right+1)/2;
        TreeNode node = new TreeNode(nums[mid]);
        node.left = arrayToBST(nums, left, mid-1);
        node.right = arrayToBST(nums, mid+1, right);

        return node;
    }
}