L 333. Largest BST Subtree ------M

 Given the root of a binary tree, find the largest subtree, which is also a Binary Search Tree (BST), where the largest means subtree has the largest number of nodes.

A Binary Search Tree (BST) is a tree in which all the nodes follow the below-mentioned properties:

• The left subtree values are less than the value of their parent (root) node's value.
• The right subtree values are greater than the value of their parent (root) node's value.

Note: A subtree must include all of its descendants.

Follow up: Can you figure out ways to solve it with O(n) time complexity?

 

Example 1:

Input: root = [10,5,15,1,8,null,7]
Output: 3
Explanation: The Largest BST Subtree in this case is the highlighted one. The return value is the subtree's size, which is 3.

Example 2:

Input: root = [4,2,7,2,3,5,null,2,null,null,null,null,null,1]
Output: 2

 

Constraints:

• The number of nodes in the tree is in the range [0, 104].
• -104 <= Node.val <= 104

A:

这个一开始理解错误， 以为是要 root value with largest mean of BST呢

其实挺简单的。 就是利用 C++ 的 pass by reference 来update 最终结果

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */


class Solution {
public:
    int largestBSTSubtree(TreeNode* root) {
        if (not root) // shall never hit
            return 0;
        int largestCount = 0;

        int minVal = INT_MAX, maxVal = INT_MIN;
        int count = 0;
        helper(root, minVal, maxVal, count, largestCount);
        return largestCount;
    }
private:
    bool helper(TreeNode* root, int& minValue, int& maxValue, int& count, int& largestCount) {
        if (not root) {
            return true;
        }
        int leftMin = INT_MAX, leftMax = INT_MIN;
        int leftCount = 0;
        bool isLeftBST = helper(root->left, leftMin, leftMax, leftCount,  largestCount);

        int rightMin = INT_MAX, rightMax = INT_MIN;
        int rightCount = 0;
        bool isRightBST = helper(root->right, rightMin, rightMax, rightCount, largestCount);

        if (isLeftBST && isRightBST && root->val > leftMax && root->val < rightMin) {
            count = 1 + leftCount + rightCount;
            if (count > largestCount) {
                largestCount = count;
            }
            minValue = root->left ? leftMin : root->val;
            maxValue = root->right? rightMax : root->val;
            return true;
        }
        return false;
    }
};