leetcode98 - Validate Binary Search Tree - medium

Given a binary tree, determine if it is a valid binary search tree (BST).

Assume a BST is defined as follows:

* The left subtree of a node contains only nodes with keys less than the node's key.

* The right subtree of a node contains only nodes with keys greater than the node's key.

* Both the left and right subtrees must also be binary search trees.

Example 1:

Input:

    2

   / \

  1   3

Output: true

Example 2:

    5

   / \

  1   4

     / \

    3   6

Output: false

Explanation: The input is: [5,1,4,null,null,3,6]. The root node's value

             is 5 but its right child's value is 4. 

 

 

法1.O(n) O(h) 利用性质：左边所有节点的值要比当前节点小，右边所有节点的值要比当前节点大。

那么就有想法，你每遍历到一个节点就对孩子们可取范围多加了一层限制。

递归函数头为：private boolean validateRange(TreeNode root, Integer minVal, Integer maxVal)

细节：

1.用Integer的包装类型的原因是，使用null代表无限制，你可以取无穷大。比如root的右边应该是(root.val, 无穷)的。

2.递归的时候需要根据当前根节点更新左右子树的范围。

 

法2.O(n) O(h) 利用性质：中序遍历是递增序列。（注意需要在左<中<右的条件下可以用这个检验。如果是左<=中<右，20- 20(root）和20(root) - 20两个中序遍历都是20，20，但前者是BST后者不是BST。)

实现1：分治法，验证合法范围。O(n), O(h)

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public boolean isValidBST(TreeNode root) {
        return validateRange(root, null, null);
    }
    
    private boolean validateRange(TreeNode root, Integer minVal, Integer maxVal) {
        if (root == null) return true;
        if (minVal != null && root.val <= minVal) return false;
        if (maxVal != null && root.val >= maxVal) return false;
        return validateRange(root.left, minVal, root.val) && validateRange(root.right, root.val, maxVal);
    }
}

实现2：recursive版中序遍历 O(n), O(h)

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public boolean isValidBST(TreeNode root) {
        List<Integer> list = new ArrayList<>();
        getInOrderResult(root, list);
        for (int i = 1; i < list.size(); i++) {
            if (list.get(i) <= list.get(i - 1)) {
                return false;
            }
        }
        return true;
    }
    
    private void getInOrderResult(TreeNode root, List<Integer> list) {
        if (root == null) {
            return;
        }
        getInOrderResult(root.left, list);
        list.add(root.val);
        getInOrderResult(root.right, list);
    }
}

实现3：iterative版中序遍历 O(n), O(h)

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public boolean isValidBST(TreeNode root) {
        // P1: null 是BST。
        List<Integer> list = new ArrayList<>();
        Stack<TreeNode> stack = new Stack<>();
        TreeNode crt = root;
        
        while (!stack.isEmpty() || crt != null) {
            while (crt != null) {
                stack.push(crt);
                crt = crt.left;
            }
            crt = stack.pop();
            list.add(crt.val);
            // P2: 不管crt.right是不是null都要赋值。
            crt = crt.right;
        }
        
        for (int i = 1; i < list.size(); i++) {
            if (list.get(i) <= list.get(i - 1)) {
                return false;
            }
        }
        return true;
    }
}