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  • LC.98.Validate Binary Search Tree

    https://leetcode.com/problems/validate-binary-search-tree/description/
    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:
    2
    /
    1 3
    Binary tree [2,1,3], return true.
    Example 2:
    1
    /
    2 3
    Binary tree [1,2,3], return false.

    note: by the definition, here it mentions:
    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.
    time: o(n)
    space: o(n)
     1 public boolean isValidBST(TreeNode root) {
     2         if (root == null ) return true;
     3         return helper(root, Long.MIN_VALUE, Long.MAX_VALUE);
     4     }
     5 
     6     private boolean helper(TreeNode root, long minValue, long maxValue) {
     7         if (root == null) return true ;
     8         if (root.val >=maxValue || root.val <= minValue){
     9             return false ;
    10         }
    11         return helper(root.left, minValue, root.val) && helper(root.right, root.val, maxValue);
    12     }
    
    




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  • 原文地址:https://www.cnblogs.com/davidnyc/p/8492027.html
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