Validate Binary Search Tree

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.

1.每次dfs传入下界和上界。

bool check(TreeNode *node, int leftVal, int rightVal){    if (node == NULL)        return true;    if(leftVal < node->val && node->val < rightVal &&       check(node->left, leftVal, node->val) && check(node->right, node->val, rightVal))        return true;    else        return false;}bool isValidBST(TreeNode *root){    return check(root, INT_MIN, INT_MAX);}

2.中序遍历，记录pre的值，注意用引用传递

bool isBin = true;void inOrder(TreeNode *root, int &pre){    if (!root )        return;    inOrder(root->left, pre);    if ( pre >= root->val)        isBin = false;    pre = root->val;    inOrder(root->right, pre);}bool isValidBST(TreeNode *root){    int pre = INT_MIN;    inOrder(root, pre);    return isBin;}