leetcode — validate-binary-search-tree

import apple.laf.JRSUIUtils;

/**
 * Source : https://oj.leetcode.com/problems/validate-binary-search-tree/
 *
 *
 *
 * 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.
 *
 * confused what "{1,#,2,3}" means? > read more on how binary tree is serialized on OJ.
 *
 * OJ's Binary Tree Serialization:
 *
 * The serialization of a binary tree follows a level order traversal, where '#' signifies
 * a path terminator where no node exists below.
 *
 * Here's an example:
 *
 *    1
 *   / \
 *  2   3
 *     /
 *    4
 *     \
 *      5
 *
 * The above binary tree is serialized as "{1,2,3,#,#,4,#,#,5}".
 */
public class ValidateBinarySearchTree {

    public boolean validate (TreeNode root) {
        return isValid(root, Integer.MIN_VALUE, Integer.MAX_VALUE);
    }

    /**
     * 先判断根节点是否满足 root.value > min && root.value < max，如果满足，再递归判断左右子树
     *
     * @param root
     * @param min
     * @param max
     * @return
     */
    public boolean isValid (TreeNode root, int min, int max) {
        if (root == null) {
            return true;
        }
        if (root.value > max || root.value < min) {
            return false;
        }
        return isValid(root.leftChild, min, root.value) && isValid(root.rightChild, root.value, max);
    }

    /**
     * 二叉搜索树的中序遍历结果是单调递增的，所以中序遍历的时候当前节点值大于上一个节点的值
     * 注意：这里每次递归会改变lastMax的值，需要保存下来，所以这里需要一个类似指针的变量，不能直接使用Integer等包装类型
     *
     * @param root
     * @param lastMax
     * @return
     */
    public boolean isValidByInorder (TreeNode root, TreeNode lastMax) {
        if (root == null) {
            return true;
        }
        if (!isValidByInorder(root.leftChild, lastMax)) {
            return false;
        }
        if (lastMax.value >= root.value) {
            return false;
        }
        lastMax.value = root.value;
        return isValidByInorder(root.rightChild, lastMax);
    }
    public boolean validate1 (TreeNode root) {
        TreeNode lastMax = new TreeNode(Integer.MIN_VALUE);
        return isValidByInorder(root, lastMax);
    }

    public TreeNode createTree (char[] treeArr) {
        TreeNode[] tree = new TreeNode[treeArr.length];
        for (int i = 0; i < treeArr.length; i++) {
            if (treeArr[i] == '#') {
                tree[i] = null;
                continue;
            }
            tree[i] = new TreeNode(treeArr[i]-'0');
        }
        int pos = 0;
        for (int i = 0; i < treeArr.length && pos < treeArr.length-1; i++) {
            if (tree[i] != null) {
                tree[i].leftChild = tree[++pos];
                if (pos < treeArr.length-1) {
                    tree[i].rightChild = tree[++pos];
                }
            }
        }
        return tree[0];
    }

    private class TreeNode {
        TreeNode leftChild;
        TreeNode rightChild;
        int value;

        public TreeNode(int value) {
            this.value = value;
        }

        public TreeNode() {
        }
    }

    public static void main(String[] args) {
        ValidateBinarySearchTree validateBinarySearchTree = new ValidateBinarySearchTree();
        System.out.println(validateBinarySearchTree.validate(validateBinarySearchTree.createTree(new char[]{'1','2','3','#','#','4','#','#','5'})) + "-------false");
        System.out.println(validateBinarySearchTree.validate(validateBinarySearchTree.createTree(new char[]{'3','1','4'})) + "-------true");
        System.out.println(validateBinarySearchTree.validate(validateBinarySearchTree.createTree(new char[]{'3','2','4','1','#'})) + "-------true");

        System.out.println();
        System.out.println(validateBinarySearchTree.validate1(validateBinarySearchTree.createTree(new char[]{'1','2','3','#','#','4','#','#','5'})) + "-------false");
        System.out.println(validateBinarySearchTree.validate1(validateBinarySearchTree.createTree(new char[]{'3','1','4'})) + "-------true");
        System.out.println(validateBinarySearchTree.validate1(validateBinarySearchTree.createTree(new char[]{'3','2','4','1','#'})) + "-------true");
    }
}