LeetCode - Validate Binary Search Tree

題目：

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.

思路：

1) 遞歸，兩邊夾

package bst;

public class ValidateBinarySearchTree {

    public boolean isValidBST(TreeNode root) {
        return isValidBST(root, false, Integer.MIN_VALUE, false, Integer.MAX_VALUE);
    }
    
    private boolean isValidBST(TreeNode root, boolean reachMin, int min, boolean reachMax, int max) {
        if (root == null) return true;
        int value = root.val;
        if (value < min || value > max) return false;
        
        if (!reachMin && value == Integer.MIN_VALUE) {
            if (root.left != null) return false;
            return isValidBST(root.right, true, value, reachMax, max);            
        } else if (!reachMax && value == Integer.MAX_VALUE) {
            if (root.right != null) return false;            
            return isValidBST(root.left, reachMin, min, true, value);
        } else if (value > min && value < max) { 
            return isValidBST(root.left, reachMin, min, reachMax, value) && 
                    isValidBST(root.right, reachMin, value, reachMax, max);        
        }
        
        return false;
    }
    
    public static void main(String[] args) {
        // TODO Auto-generated method stub

    }

}

2) 類似於中序遍歷

package bst;

public class ValidateBinarySearchTree {

    private boolean firstNode = true;
    
    private int lastValue = 0;
    
    public boolean isValidBST(TreeNode root) {
        if (root == null) return true;
        if (!isValidBST(root.left)) return false;
        
        if (!firstNode && root.val <= lastValue) return false;
        firstNode = false;
        lastValue = root.val;
        
        if (!isValidBST(root.right)) return false;
        
        return true;
    }
    
    public static void main(String[] args) {
        // TODO Auto-generated method stub

    }

}