驗證二叉搜索樹

原題

　　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.

題目大意

　　驗證二叉搜索樹

解題思路

　　對二叉搜索樹進行中序遍歷，結果按順序保存起來，對於二叉搜索樹中序遍歷其結果有一個從小到大的排列的序列，而且沒有重重元素，由此能夠判斷一棵樹是不是二叉搜索樹。

代碼實現

樹結點類

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

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算法實現類

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樹必必是有序的
     * @param root
     */
    public void inOrder(TreeNode root) {
        if (root != null) {
            inOrder(root.left);
            stack.push(root.val);
            inOrder(root.right);
        }
    }
}