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

Quesion

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:

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.

Solution

采用迭代处理

基本原理就是中序遍历过程，期间只要判断是否存在不满足条件，也就是有序

Runtime: 1 ms, faster than 36.18% of Java online submissions for Validate Binary Search Tree.
Memory Usage: 40.5 MB, less than 5.01% of Java online submissions for Validate Binary Search Tree.
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public boolean isValidBST(TreeNode root) {
    return isValid(root, Long.MIN_VALUE, Long.MAX_VALUE);
}

private boolean isValid(TreeNode root, Long min, Long max) {
    if (root == null) return true;
    if (root.val <= min || root.val >= max) return false;
    return isValid(root.left, min, (long)root.val) && isValid(root.right, (long)root.val, max);
}

采用stack，非迭代

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