[google面试题]Write a function to determine whether a given binary tree of distinct integers is a valid binary search tree.

Good Answer:Note that it's not enough to write are cursive function that just checks if the left and right nodes of each node are less than and greater than the current node(and calls that recursively).You need to make sure that all the nodes of the subtree starting at your current node are within the valid range of values allowed by the current node's ancestors.Therefore you can solve this recursively by writing a helper function that accepts a current node,the smallest allowed value,and the largest allowed value for that subtree.An example of this is the following(in Java):boolean isValid(Node root) { return isValidHelper(root, Integer.MIN_VALUE,Integer.MAX_VALUE);}boolean isValidHelper(Node curr, int min,int max) { if(curr.left != null) { if(curr.left.value < min ||!isValidHelper(curr.left, min, curr.value)) return false; } if(curr.right != null) { if(curr.right.value > max ||!isValidHelper(curr.right, curr.value,max)) return false; } return true;}The running time of this algorithm is O(n).