LeetCode Largest BST Subtree

原题链接在这里：https://leetcode.com/problems/largest-bst-subtree/

Given a binary tree, find the largest subtree which is a Binary Search Tree (BST), where largest means subtree with largest number of nodes in it.

Note:
A subtree must include all of its descendants.
Here's an example:

10
/ \
5  15
/ \   \
1   8   7

The Largest BST Subtree in this case is the highlighted one.
The return value is the subtree's size, which is 3.

Show Hint

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

采用bottom-up的方法，简历新的class, 用来存储

当前节点为root的subtree是否是BST

若是，最小val 和最大val.

size是当前subtree的大小。

然后从下到上更新，若是中间过程中size 比 res大，就更新res.

Time Complexity: O(n). 每个点不会访问超过两遍. Space: O(logn). Recursion stack space.

AC Java:

/**
* Definition for a binary tree node.
* public class TreeNode {
*     int val;
*     TreeNode left;
*     TreeNode right;
*     TreeNode(int x) { val = x; }
* }
*/
public class Solution {
public int largestBSTSubtree(TreeNode root) {
int [] res = {0};
helper(root, res);
return res[0];
}

private Node helper(TreeNode root, int [] res){
Node cur = new Node();
if(root == null){
cur.isBST = true;
return cur;
}
Node left = helper(root.left, res);
Node right = helper(root.right, res);
if(left.isBST && root.val > left.max && right.isBST && root.val < right.min){
cur.isBST = true;
cur.min = Math.min(root.val, left.min);
cur.max = Math.max(root.val, right.max);
cur.size = left.size + right.size + 1;
if(cur.size > res[0]){
res[0] = cur.size;
}
}
return cur;
}
}

class Node{
boolean isBST;
int min;
int max;
int size;
public Node(){
isBST = false;
min = Integer.MAX_VALUE;
max = Integer.MIN_VALUE;
size = 0;
}
}