0098.cpp

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
  bool isValidBST(TreeNode *root) {
    // return func1(root);
    return func2(root);
  }

  // Wrong solution: [10,5,15,null,null,6,20]
  bool func1(TreeNode *root) {
    if (root == NULL) {
      return true;
    }
    if (root->left != NULL) {
      if (root->val <= root->left->val) {
        return false;
      }
      if (func1(root->left) == false) {
        return false;
      }
    }
    if (root->right != NULL) {
      if (root->val >= root->right->val) {
        return false;
      }
      if (func1(root->right) == false) {
        return false;
      }
    }
    return true;
  }

  // Complexity: T(n) = 2T(n/2) + O(n) => O(nlogn)
  bool func2(TreeNode *root) {
    if (root == NULL) {
      return true;
    }
    int left_max = 0, right_min = 0;
    bool ret = true;
    if (root->left) {
      left_max = getLeftMax(root->left);
      ret = ret && root->val > left_max;
    }
    if (root->right) {
      right_min = getRightMin(root->right);
      ret = ret && root->val < right_min;
    }
    if (ret == false) {
      return false;
    }
    return func2(root->left) && func2(root->right);
  }
  int getLeftMax(TreeNode *root) {
    while (root->right) {
      root = root->right;
    }
    return root->val;
  }
  int getRightMin(TreeNode *root) {
    while (root->left) {
      root = root->left;
    }
    return root->val;
  }
};