leetcode 98 Valid Binary Search Tree
z
 2018-08-16 10:10:22  

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:

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5
Input:
    2
   / \
  1   3
Output: true

Example 2:

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    5
   / \
  1   4
     / \
    3   6
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.
  • 使用一个stack，每遇到一个node，一直向左走，将经过的元素加入栈中
  • pop出一个元素，使之为root，标记该root为pre，继续下一次向左走，此时，比较每一个经过的值和pre的大小
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class Solution {
    public boolean isValidBST(TreeNode root) {
        if(root == null){
            return true;
        }
        Stack<TreeNode> s = new Stack<>();
        TreeNode pre = null;
        while(!s.isEmpty() || root != null){
            while(root != null){
                s.add(root);
                root = root.left;
            }
            
            root = s.pop();
            if (pre != null && pre.val >= root.val){
                return false;
            }
            pre = root;
            root = root.right;
        }
        return true;

    }
}