LeetCode -- Validate Binary Search Tree

LeetCode -- Validate Binary Search Tree

題目描述：


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.


就是判斷一個二叉樹是否為搜索二叉樹。


搜索二叉樹的定義為，左子樹的每個節點都小於當前節點；右子樹的每個節點都大於當前節點。


思路：中序遍歷節點存入數組，再判斷數組是否為遞增（不包括相等）。


實現代碼：

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left;
 *     public TreeNode right;
 *     public TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public bool IsValidBST(TreeNode root) 
    {
        var result = new List();
    	Travel(root, ref result);
    	
    	for(var i = 0;i < result.Count - 1; i++){
    		if(result[i+1] <= result[i]){
    			return false;
    		}
    	}
    	
    	return true;
    }


private void Travel(TreeNode node ,ref List result)
{
	if(node == null){
		return;
	}
	
	Travel(node.left, ref result);
	result.Add(node.val);
	Travel(node.right, ref result);
}
}