LeetCode热题100-41、42题

41、有序数组转为二叉搜索树

给你一个整数数组 nums ，其中元素已经按 升序 排列，请你将其转换为一棵 平衡 二叉搜索树。

/**
 * 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 helper(nums,0,nums.length - 1);
    }

    public TreeNode helper(int[] nums,int left,int right) {
        if (left > right) {
            return null;
        }

        int mid = (left + right) / 2;
        TreeNode root = new TreeNode(nums[mid]);
        root.left = helper(nums,left,mid - 1);
        root.right = helper(nums,mid + 1,right);
        return root;
    }
}

42、验证二叉搜索树

/**
 * 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 boolean isValidBST(TreeNode root) {
        return isValidBST(root,Long.MIN_VALUE,Long.MAX_VALUE);
    }

    public boolean isValidBST(TreeNode node,long lower,long upper) {
        if (node == null) {
            return true;
        }

        if (node.val <= lower || node.val >= upper) {
            return false;
        }

        return isValidBST(node.left,lower,node.val) && isValidBST(node.right,node.val,upper);
    }
}