98

98.validate-binary-search-tree.rb

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+# -*- coding: utf-8 -*-
+#
+# @lc app=leetcode id=98 lang=ruby
+#
+# [98] Validate Binary Search Tree
+#
+# https://leetcode.com/problems/validate-binary-search-tree/description/
+#
+# 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.
+#
+# Example 1:
+#
+# Input:
+# ⁠   2
+# ⁠  / \
+# ⁠ 1   3
+# Output: true
+#
+# Example 2:
+#
+# Input:
+# ⁠   5
+# ⁠  / \
+# ⁠ 1   4
+# / \
+# 3   6
+# Output: false
+# Explanation: The input is: [5,1,4,null,null,3,6]. The root node's
+# value
+# is 5 but its right child's value is 4.
+#
+# Definition for a binary tree node.
+# class TreeNode
+#     attr_accessor :val, :left, :right
+#     def initialize(val)
+#         @val = val
+#         @left, @right = nil, nil
+#     end
+# end
+
+
+# In-order traversal the tree and check if it's sorted
+
+# @param {TreeNode} root
+# @return {Boolean}
+def is_valid_bst_a(root)
+  inorder_traversal(root).each_cons(2).all? { |p, q| (p <=> q) < 0 }
+end
+
+def inorder_traversal(root)
+  [].tap { |s| helper_a(root, s) }
+end
+
+def helper_a(root, a)
+  return if root.nil?
+  helper_a(root.left, a)
+  a << root.val
+  helper_a(root.right, a)
+end
+
+
+# Save space by checking only the current and previous node
+# Ruby is call-by-value so we have to use a single element array to store previous
+# Given:
+#     3
+#    / \
+#   1   4
+#  / \
+# 0   2
+#
+#calls      in-order    previous
+#3,a        []          []
+#  1,a      []          []
+#    0,a    []          []
+#      a<<0 [0]         [0]  (*special*)
+#    a<<1   [0,1]       [1]  (0 < 1)
+#    2,a    [0,1]       [1]
+#      a<<2 [0,1,2]     [2]  (1 < 2)
+#  a<<3     [0,1,2,3]   [3]  (2 < 3)
+#  4,a      [0,1,2,3]   [3]
+#    a<<4   [0,1,2,3,4] [4]  (3 < 4)
+
+def is_valid_bst_b(root)
+  helper_b(root)
+end
+
+def helper_b(root, prev=[])
+  return true if root.nil?
+  helper_b(root.left, prev) && checker_b(root, prev) && helper_b(root.right, prev)
+end
+
+def checker_b(node, prev)
+  return false if !prev.empty? && node.val <= prev[0]
+  prev[0] = node.val and return true
+end
+
+
+# Narrowing min,max for each node
+# Given:
+#                3(-inf,+inf)
+#               / \
+#      (-inf,3)1   4(3,+inf)
+#             / \
+#    (-inf,1)0   2(1,3)
+
+def is_valid_bst(root)
+  helper(root, -Float::INFINITY, Float::INFINITY)
+end
+
+def helper(node, low, hi)
+  return true if node.nil?
+  return false unless node.val > low && node.val < hi
+  helper(node.left, low, node.val) && helper(node.right, node.val, hi)
+end