Problem
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.
Example: Input: [10,5,15,1,8,null,7] 10 / 5 15 / 1 8 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.
Follow up:
Can you figure out ways to solve it with O(n) time complexity?
First try:
class Solution { public int largestBSTSubtree(TreeNode root) { if (root == null) return 0; if (isBST(root)) return largestBSTSubtree(root.left)+largestBSTSubtree(root.right)+1; return Math.max(largestBSTSubtree(root.left), largestBSTSubtree(root.right)); } private boolean isBST(TreeNode root) { if (root == null) return true; if (root.left != null && findMax(root.left) >= root.val) return false; if (root.right != null && findMin(root.right) <= root.val) return false; if (isBST(root.left) && isBST(root.right)) return true; return false; } private int findMax(TreeNode root) { while (root.right != null) root = root.right; return root.val; } private int findMin(TreeNode root) { while (root.left != null) root = root.left; return root.val; } }
Optimized:
class Solution { public int largestBSTSubtree(TreeNode root) { if (root == null) return 0; if (isBST(root, null, null)) return largestBSTSubtree(root.left)+largestBSTSubtree(root.right)+1; return Math.max(largestBSTSubtree(root.left), largestBSTSubtree(root.right)); } private boolean isBST(TreeNode root, Integer min, Integer max) { if (root == null) return true; if (min != null && min >= root.val) return false; if (max != null && max <= root.val) return false; if (isBST(root.left, min, root.val) && isBST(root.right, root.val, max)) return true; return false; } }
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摘要:复杂度思路考虑对于每一个节点来说,能组成的的。那么并且所以我们需要两个返回值,一个是这个是不是,另一个是当前的能组成的最大的值。代码这个能构成一个这个不能构成一个 LeetCode[333] Largest BST Subtree Given a binary tree, find the largest subtree which is a Binary SearchTree (B...
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 ...
10 / 5 15 / 1 8 7 return 3 public class Solution { public int largestBSTSubtree(TreeNode root) { if(root == null) return 0; int[] res = recursive(root); ...
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