LeetCode百题【将有序数组转换为二叉搜索树】

题目描述

题目描述.png

解法

分治法

选出根节点
比根节点小的值放入左子树
比根节点大的值放入右子树
对于左右子树同上，进行递归

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public TreeNode sortedArrayToBST(int[] nums) {
        return BuildTree(nums,0,nums.length-1);
    }
    public TreeNode BuildTree(int[] nums,int left,int right){
        if(right<left){
            return null;
        }//考虑叶节点子树为空的情况
        int mid=(left+right)/2;
        TreeNode root=new TreeNode(nums[mid]);
        root.left=BuildTree(nums,left,mid-1);//对于左节点，left没有变过，通过调节right调整mid改变root的位置
        root.right=BuildTree(nums,mid+1,right);//对于右节点，right没有变过，通过调节left调整mid改变root的位置
        return root;
    }
}