Leetcode之中序遍历迭代方法解决多个问题

最新推荐文章于 2024-07-01 14:36:43 发布

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最新推荐文章于 2024-07-01 14:36:43 发布

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分类专栏： Leetcode 文章标签：中序遍历 BST 二叉搜索树

本文链接：https://blog.youkuaiyun.com/qq_21815981/article/details/80556666

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本文深入探讨了二叉树的几种关键算法实现：中序遍历、查找二叉搜索树中第K小元素及验证二叉搜索树的有效性。通过递归与迭代两种方式展示算法细节，并提供完整的Java代码实现。

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94. Binary Tree Inorder Traversal

Given a binary tree, return the inorder traversal of its nodes’ values.

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<Integer> inorderTraversal(TreeNode root) {
        List<Integer> list = new ArrayList<>();
        if(root == null) return list;
        Stack<TreeNode> stack = new Stack<>();
        while(root != null || !stack.empty()){
            while(root != null){
                stack.push(root);
                root = root.left;
            }
            root = stack.pop();
            list.add(root.val);
            root = root.right;    
        }
        return list;
    }
}

230. Kth Smallest Element in a BST

Given a binary search tree, write a function kthSmallest to find the kth smallest element in it.

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public int kthSmallest(TreeNode root, int k) {
        Stack<TreeNode> stack = new Stack<>();
        while(root != null || !stack.isEmpty()) {
            while(root != null) {
                stack.push(root);    
                root = root.left;   
            } 
            root = stack.pop();
            if(--k == 0) break;
            root = root.right;
        }
     return root.val;
    }
}

98. Validate Binary Search Tree

Given a binary tree, determine if it is a valid binary search tree (BST).

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public boolean isValidBST(TreeNode root) {
        if (root == null) return true;
        Stack<TreeNode> stack = new Stack<>();
        TreeNode pre = null;
        while (root != null || !stack.isEmpty()) {
            while (root != null) {
                stack.push(root);
                root = root.left;
            }
            root = stack.pop();
            if(pre != null && root.val <= pre.val) 
                return false;
            pre = root;
            root = root.right;
        }
        return true;
    }
}