验证二叉搜索树

原题

　　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.

题目大意

　　验证二叉搜索树

解题思路

　　对二叉搜索树进行中序遍历，结果按顺序保存起来，对于二叉搜索树中序遍历其结果有一个从小到大的排列的序列，而且没有重重元素，由此能够判断一棵树是不是二叉搜索树。

代码实现

树结点类

public class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;
    TreeNode(int x) { val = x; }
}

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算法实现类

import java.util.Stack;

public class Solution {

    private Stack<Integer> stack;
    public boolean isValidBST(TreeNode root) {

        if (root == null) {
            return true;
        }

        stack = new Stack<>();
        inOrder(root);

        int i = stack.pop();
        int j;
        while (!stack.isEmpty()) {
            j = stack.pop();
            if (i <= j) {
                return false;
            }

            i = j;
        }

        return true;
    }

    /**
     * 若是是一棵二叉查找树必必是有序的
     * @param root
     */
    public void inOrder(TreeNode root) {
        if (root != null) {
            inOrder(root.left);
            stack.push(root.val);
            inOrder(root.right);
        }
    }
}